Normally, one would have a chuckle at the thought of good, patriotic Missourians borrowing ideas from the French :pan:, but this latest traffic innovation:
http://www.modot.org/springfield/major_projects/Greene/I-44andRoute13.html (http://www.modot.org/springfield/major_projects/Greene/I-44andRoute13.html)
sounds like it has quite a bit of merit to it :clap: MoDOT is apparently redoing 4 interchanges using the Diverging Diamond method, and UDOT is reworking another interchange on I-15 this way, too.
Would suck to be a pedestrian crossing this interchange, though, as the pedestrian(s) would end up effectively waiting in the middle of the street to cross both ends of it :-o
Looks like an SPUI without everyone being affected by the stoplight
I like this design. It's still a simple SPUI (essentially) with crossing lanes that eliminate dangerous left turns across busy highways
that actually looks like a nice idea for an interchange!
The idea is interesting, but I'm afraid it's a bit too unorthodox to actually work. You have to drive in the "wrong" direction.
Like any other new traffic pattern (such as roundabouts or SPUI's at one time), this would take a bit of getting used to. I like it, though, in that the signal operations are simplified. Conventional diamonds are terrible when left turn volumes to the ramps are high.
Ooh... :wow:
Hmm... That sounds rather clever, at least on paper. However, it would take some getting used to, so MODOT may need to do some kind of massive publicity campaign.
Quite interesting. I like it, I like it when they try new things
The following was originally posted by me to an MTR thread about DDIs.
My take is that the SPUI should be used at high traffic volume
interchanges, especially where lots of left-turn traffic is expected.
Traditional diamonds are best for slower interchanges such as rural
locations, and interchanges where most of the traffic is right-turning
(so that the benefits of a SPUI would be negligible, and little
"corner-cut" ramps could be used instead.
The DDI seems to me to be a solution in search of a problem. Having
two "cross-over" locations is just asking for trouble...you're bound
to get Grandpa Artie all confused and driving into incoming traffic.
I'm not sure if the additional thought required of drivers is too much
for the safety improvement of not having to turn left. It is inferior
to the SPUI because it could still require waits at two signals,
whereas the SPUI requires only one (though left-turning traffic from
the ramps will only have to have one, they will have a yield to
contend with, which can potentially be as long of a wait as a
stoplight if traffic is heavy). About the only good thing about the
DDI is that it's cheaper than a SPUI.
Spoke with a couple of people at the Grand Prairie Parkway DDI today while photographing it on the drive from the Quad Cities to Omaha. They were doing some research on a recent accident involving a wrong way driver, which claimed the lives of two police officers. The accident occurred on I-80 just to the west of this DDI. While discussing the interchange, they asked what were some of the opinions drivers had of them, and I shared some of the opinions we road enthusiasts have of them. They asked if any of the confusion involving DDI's have resulted in either a major accident or a fatality. I cannot think of any off-hand, does anyone here know of any such incident?
A former member of this forum used to call them "death diamonds," but I have never heard of any issues with DDIs.
Coincidentally, just this morning there was a serious wrong-way driver accident here in South Bend: http://www.wsbt.com/news/local/stretch-of-the-bypass-closed-after-wrongway-crash/38741088 (http://www.wsbt.com/news/local/stretch-of-the-bypass-closed-after-wrongway-crash/38741088)
The pertinence to this discussion is that there are no DDI interchanges in the area of the accident. Though her entrance point hasn't yet been reported, the most likely one is SR-23. a standard diamond. My point is that you don't need a DDI to have a wrong-way driver. It would be interesting to see a statistical comparison of DDI's vs. other interchange designs, regarding incidents of wrong-way driving.
Wow, it's kinda cool to look back at the posts from when DDI's were just an idea on paper 7 years ago!
Re: wrong-way driving: I was under the impression that signage and road geometry at DDI's were usually such that head-on collisions are minimized. Unless something was done wrong, the crossover intersections should be at an angle wide enough that you would have to make a right turn to go the wrong way on the road. In my experience (however limited it may be), it just feels like the road splits into a pair of one-way streets for the duration of the interchange.
Unfortunately, the DDI craze has led to them being overbuilt across the country in locations where they weren't a good fit. There are examples of DDI's replacing Parclos. A simple Parclo could prevent the need for diverging movements along the arterial (leading to fewer conflict points) but engineers are enamored with the DDI. But i think we are starting to get back to earth. DOTs that have a lot of experience with them (UDOT) have admitted in their published DDI reports that they haven't performed as well as they had hoped. This is the first line found in the Utah DDI guide. I think it speaks for itself.
QuoteAs new innovations are introduced and applied, they sometimes struggle for initial acceptance but then gain in popularity until they are often over-applied.
Just drove through the new DDI to the west of US 77 on I-80 outside Lincoln. This one is replacing a parclo interchange with Nebraska L55K.
A full cloverleaf interchange between I-74/US 6 and IL 5 (John Deere Expressway) in Moline is under study to be converted to a DDI as well. Some of them are overkill while others seem to fall under the road diet category to me.
Quote from: tradephoric on March 29, 2016, 05:47:24 PM
Unfortunately, the DDI craze has led to them being overbuilt across the country in locations where they weren't a good fit. There are examples of DDI's replacing Parclos. A simple Parclo could prevent the need for diverging movements along the arterial (leading to fewer conflict points) but engineers are enamored with the DDI. But i think we are starting to get back to earth. DOTs that have a lot of experience with them (UDOT) have admitted in their published DDI reports that they haven't performed as well as they had hoped. This is the first line found in the Utah DDI guide. I think it speaks for itself.
QuoteAs new innovations are introduced and applied, they sometimes struggle for initial acceptance but then gain in popularity until they are often over-applied.
I agree DDIs can only handle so much traffic, just like all interchanges, and aren't always a good fit. For example Vdot will soon be constructing two DDIs on I-66, one at Exit 40 in Haymarket and one at Exit 62 in Vienna. I think the one in Haymarket is a great idea while the one in Vienna may be well overburdened and cause even more delays. In short even DDIs may not always be the best option
Here's a link to the I-74/US6/IL5 cloverleaf interchange in Moline that Alex referenced. I don't hide the fact that i'm not a big fan of the DDI. But to all the DDI proponents, do you really think this is right interchange to convert to a DDI?
https://www.google.com/maps/@41.4714764,-90.4945593,410m/data=!3m1!1e3
Quote from: tradephoric on March 29, 2016, 07:09:16 PM
Here's a link to the I-74/US6/IL5 cloverleaf interchange in Moline that Alex referenced. I don't hide the fact that i'm not a big fan of the DDI. But to all the DDI proponents, do you really think this is right interchange to convert to a DDI?
https://www.google.com/maps/@41.4714764,-90.4945593,410m/data=!3m1!1e3
Considering recent upgrades to IL 5 just to the east to enhance it as an expressway, I do not think it is the right interchange here.
September 2015 story on the DDI: http://qctimes.com/news/local/changes-being-considered-for-i--john-deere-road-interchange/article_9fbde01d-c10c-5546-8020-7eb7a7e41c3b.html
Quote from: Alex on March 29, 2016, 07:26:58 PM
Quote from: tradephoric on March 29, 2016, 07:09:16 PM
Here's a link to the I-74/US6/IL5 cloverleaf interchange in Moline that Alex referenced. I don't hide the fact that i'm not a big fan of the DDI. But to all the DDI proponents, do you really think this is right interchange to convert to a DDI?
https://www.google.com/maps/@41.4714764,-90.4945593,410m/data=!3m1!1e3
Considering recent upgrades to IL 5 just to the east to enhance it as an expressway, I do not think it is the right interchange here.
September 2015 story on the DDI: http://qctimes.com/news/local/changes-being-considered-for-i--john-deere-road-interchange/article_9fbde01d-c10c-5546-8020-7eb7a7e41c3b.html
It looks like there's a trailer park just to the southeast called "cloverleaf village" in reference to the interchange. Won't that be funny if it isn't a cloverleaf anymore! Will they have to change their name? :D
Quote from: paulthemapguy on April 05, 2016, 12:55:47 AM
Quote from: Alex on March 29, 2016, 07:26:58 PM
Quote from: tradephoric on March 29, 2016, 07:09:16 PM
Here's a link to the I-74/US6/IL5 cloverleaf interchange in Moline that Alex referenced. I don't hide the fact that i'm not a big fan of the DDI. But to all the DDI proponents, do you really think this is right interchange to convert to a DDI?
https://www.google.com/maps/@41.4714764,-90.4945593,410m/data=!3m1!1e3
Considering recent upgrades to IL 5 just to the east to enhance it as an expressway, I do not think it is the right interchange here.
September 2015 story on the DDI: http://qctimes.com/news/local/changes-being-considered-for-i--john-deere-road-interchange/article_9fbde01d-c10c-5546-8020-7eb7a7e41c3b.html
It looks like there's a trailer park just to the southeast called "cloverleaf village" in reference to the interchange. Won't that be funny if it isn't a cloverleaf anymore! Will they have to change their name? :D
This very thing has happened a lot in New Jersey, especially when a business opens up by a traffic circle and included 'Circle' in their name. One recent example is Circle Liquors which sat at the former Somers Point Circle, which is now a regular + intersection.
Then again, NJ has a hard time giving up names as well. The jungle at 30 & 130 is still known as the Airport Circle, even though the airport closed in the 1950's. Just down the road, you can view NJDOT's self-named Collingswood Circle traffic camera...which overlooks a normal intersection.
Scott5114 nailed it on the head years ago: the primary advantage of a DDI is that it's cheap. Does anyone really wonder why Missouri of all places has been such a pioneer of them? It's because Missouri hurts for highway network funds and is eager to try new, cheaper improvements.
Sorry we've now gotten off topic from the original necro-post-question.
Bit of a revival here...
In 2013, South Africa converted the diamond interchange at the N2's junction with the R102 north of Durban to a diverging diamond interchange. The interchange is known locally as the KwaMashu interchange. It was the first diverging diamond interchange constructed in a "keep-left" country, and also the first in the southern hemisphere.
http://www.civildesigner.com/press/kwamashu.pdf --- https://goo.gl/xAHrZI
https://youtu.be/b8jO0b4wXfk
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F0UCKiRO.png&hash=07469f26936eda80c522b787bc77357309311dc4)
A new DDI at University & I-75 in Michigan is signed for 25 mph. Directly upstream and downstream of the DDI the arterial is signed for 45 mph. Subdivision streets are signed for 25 mph. Good thing drivers travel nearly 2500 feet through this suburban DDI at 25 mph. Maybe the designers just want drivers to slow down so they can admire their fabulous DDI. We are living in bubble-boy world where high speed suburban arterials are signed for 25 mph.
The DDI at I-270 and Roberts Rd in Columbus works very well. It moves traffic much better than the conventional diamond interchange it replaced. I do not believe a SPUI would offer any significant operational advantages over a DDI here. I have not heard of any wrong-way drivers at this location. Local drivers call it weird, but they seem to have accepted it more quickly than when roundabouts made their debut here.
The primary hypothetical operational weakness of a DDI is a suboptimal experience for through traffic on the arterial roadway. When the volume of that traffic flow is insignificant compared to the traffic entering and exiting the freeway, as is the case at Roberts Rd and I-270, this concern is of minimal importance.
Quote from: tradephoric on September 14, 2016, 10:23:27 AM
A new DDI at University & I-75 in Michigan is signed for 25 mph. Directly upstream and downstream of the DDI the arterial is signed for 45 mph. Subdivision streets are signed for 25 mph. Good thing drivers travel nearly 2500 feet through this suburban DDI at 25 mph. Maybe the designers just want drivers to slow down so they can admire their fabulous DDI. We are living in bubble-boy world where high speed suburban arterials are signed for 25 mph.
It takes 1.14 minutes to drive 2500 feet at 25 mph. It takes 0.63 minutes to drive 2500 feet at 45 mph. So, basically you're adding 30 seconds to traverse the intersection completely, assuming neither light is red and traffic can maintain the posted speed limit.
On the plus side, any T-bone crashes at the signalized conflict points will be much less likely to lead to deaths and injuries at 25 mph than 45 mph.
Quote from: lordsutch on September 14, 2016, 10:20:39 PM
Quote from: tradephoric on September 14, 2016, 10:23:27 AM
A new DDI at University & I-75 in Michigan is signed for 25 mph. Directly upstream and downstream of the DDI the arterial is signed for 45 mph. Subdivision streets are signed for 25 mph. Good thing drivers travel nearly 2500 feet through this suburban DDI at 25 mph. Maybe the designers just want drivers to slow down so they can admire their fabulous DDI. We are living in bubble-boy world where high speed suburban arterials are signed for 25 mph.
It takes 1.14 minutes to drive 2500 feet at 25 mph. It takes 0.63 minutes to drive 2500 feet at 45 mph. So, basically you're adding 30 seconds to traverse the intersection completely, assuming neither light is red and traffic can maintain the posted speed limit.
On the plus side, any T-bone crashes at the signalized conflict points will be much less likely to lead to deaths and injuries at 25 mph than 45 mph.
And what's the traffic volume of drivers traveling that full 2500 foot distance on University? What are the traffic volumes of drivers who make a right turn to enter I-75, drivers who make a left turn to enter I-75, drivers exiting I-75 who turn right, and drivers exiting I-75 who turn left? I suspect (because this is the most sensible reason to select a DDI over other designs) that the through traffic on University is the smallest-volume traffic movement at the interchange. Fussing about adding half a minute of travel time for a small fraction of the users of an interchange is missing the point.
Quote from: vtk on September 15, 2016, 01:23:11 AM
I suspect (because this is the most sensible reason to select a DDI over other designs) that the through traffic on University is the smallest-volume traffic movement at the interchange. Fussing about adding half a minute of travel time for a small fraction of the users of an interchange is missing the point.
I couldn't find an environmental assessment for the I-75/University DDI but ironically there is another DDI being constructed at I-75/University about 1000 miles to the south. Take a look at this model FDOT put together. Can you honestly tell me that the through traffic on University Parkway isn't a significant movement? It looks to be just as heavy if not heavier than the traffic entering I-75.
https://www.youtube.com/watch?v=24jMVZszPTY
UDOT put together an evaluation of the first DDI constructed in Utah at American Fork and I-15. Detailed turning movement counts are included in that study. If you do some simple arithmetic you will find that 37% of drivers traveling EB American Fork continue straight through the DDI and 51% of drivers traveling WB American Fork continue straight through during peak hour. Contrary to what you suspect VTK, a lot of drivers continue on the arterial at a DDI and don't enter the freeway.
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi478.photobucket.com%2Falbums%2Frr144%2Ftradephoric%2FTransportation%2520Pictures%2FRandom%2FUDOTUntitled_zpsxg1l0dlg.png&hash=4c6ef64db617ef1da80dc5cc1e7a846290d03d96)
https://www.udot.utah.gov/main/uconowner.gf?n=10172614219775523
It's difficult to provide good signal progression at closely spaced traffic signals. Providing good signal progression in one direction often leads to green-to-red signal progression in the other. Here is a video of the "arrival performance" of traffic at the Bangerter DDI in Salt Lake City, Utah. Throughout the video you see drivers on Bangerter Hwy experience green-to-red coordination while traveling straight through the DDI:
https://www.youtube.com/watch?v=RhazZu4xMeA
Another thing to consider is that DDI's often require short cycle lengths (to prevent backups on the bridge deck). This means that DDI often aren't coordinated with the surrounding signals which lead to another potential green-to-red scenario for drivers.
QuoteUDOT DIVERGING DIAMOND INTERCHANGE (DDI) OBSERVATIONS AND EXPERIENCE:
Coordination of the DDI with adjacent signals is not easily done. Most DDIs need a lower cycle length than the adjacent signals. This may result in a vehicle having to stop at both the off ramp terminal and the next adjacent signal.
Quote from: lordsutch on September 14, 2016, 10:20:39 PM
It takes 1.14 minutes to drive 2500 feet at 25 mph. It takes 0.63 minutes to drive 2500 feet at 45 mph. So, basically you're adding 30 seconds to traverse the intersection completely, assuming neither light is red and traffic can maintain the posted speed limit.
What is the likelihood of the highlighted statement being reality at a DDI?
Quote from: kphoger on September 16, 2016, 05:40:57 PM
Quote from: lordsutch on September 14, 2016, 10:20:39 PM
It takes 1.14 minutes to drive 2500 feet at 25 mph. It takes 0.63 minutes to drive 2500 feet at 45 mph. So, basically you're adding 30 seconds to traverse the intersection completely, assuming neither light is red and traffic can maintain the posted speed limit.
What is the likelihood of the highlighted statement being reality at a DDI?
Pretty low to begin with, of course.
Tradephoric, my my defense of the DDI applies only to those situations where the through arterial volume is insignificant, in which case your whole signal progression rant is insignificant. Don't rant at me for DOTs building them where they are not appropriate.
I missed signal progression talk.
Quote from: vtk on September 14, 2016, 09:27:11 PM
The primary hypothetical operational weakness of a DDI is a suboptimal experience for through traffic on the arterial roadway. When the volume of that traffic flow is insignificant compared to the traffic entering and exiting the freeway, as is the case at Roberts Rd and I-270, this concern is of minimal importance.
Here are 2030 turning movement counts for the Roberts Rd and I-270 interchange. During the AM rush, 55% of EB Roberts Rd drivers travel straight through at least one DDI signal and 24% travel straight through both DDI signals. Going WB in the AM, 53% travel straight through one DDI signal and 45% travel through both DDI signals. If you have any questions how I came up with these percentages I'll be more than happy to step you through it (I sincerely mean that).
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi478.photobucket.com%2Falbums%2Frr144%2Ftradephoric%2FTransportation%2520Pictures%2FRandom%2Frobertsroad_zpsq0uotegp.png&hash=109b78d0fe2496e6fa45e15385efb7672d449251)
http://www.morpc.org/trans/nw33270ex%20fut%20conditionssecondhalf.pdf
The percentage of through traffic at the Roberts Rd and I-270 DDI is not "insignificant" . I agree that a primary operational weakness of a DDI is a "˜suboptimal experience for through traffic on the arterial roadway' but I disagree with VTK that it's "˜hypothetical'.
Quote from: tradephoric on September 17, 2016, 11:13:18 AM
http://www.morpc.org/trans/nw33270ex%20fut%20conditionssecondhalf.pdf
To better understand the data tradephoric dug up, I visualized it by drawing each movement as a lane whose width is proportional to the traffic volume.
2003 AM/PM peak hour volumes:
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fvidthekid.info%2Fimghost%2F270rob03vol.png&hash=17d455840f4a51df6bb8c7daf6cd72ca6d2b2509)
Forecast 2030 AM/PM peak hour volumes:
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fvidthekid.info%2Fimghost%2F270rob30vol.png&hash=d62003f7311fa6c94d4b7977d665f9e0fc3721a1)
I noted that the Roberts Rd through movements in 2003 are pretty darn slender compared to some of the turning movements. Then I scratched my head for a bit. Why was the through Roberts Rd traffic forecast to increase so much? Roberts Rd doesn't really go anywhere to the east. Did the consultant assume the long-abandoned Roberts Rd / Lane Ave bridge might be built? And why does the PM NB to WB movement actually decrease from 2003 to 2030? Then I remembered traffic volume projections don't have a great track record of turning out accurate.
Then I remembered this study was done in like 2004, before the Great Recession. Remember how a few years ago a lot of folks were proclaiming we'd reached "peak VMT", as though humans will never again do as much road driving as we recently did? Then, as the economic recovery continued, people started driving more again. Traffic volumes today are probably still a bit lower than what would have been predicted for this year a decade ago, and traffic volumes in 2030 will probably maybe wind up a bit lower than what this study suggested. So, basically, I'm going to say those 2030 projections should be interpreted with a big giant asterisk.
After thinking a bit more about it, I decided those figures still don't quite illustrate my point as clearly as I'd like. Then I realized, all I really need to do is show the relative volumes of Roberts Rd through traffic and the entering & exiting traffic, with those broad categories in aggregate. So here are the pie charts:
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fvidthekid.info%2Fimghost%2F270rob03volpie.png&hash=0398f649d29a572efb3b161649d3c8746dbdec2e)
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fvidthekid.info%2Fimghost%2F270rob30volpie.png&hash=d80c6bbf295db406452ff408e9b43eebcdd4a7e2)
The Roberts Rd through traffic accounted for roughly 10% of all the traffic using that interchange in 2003. The forecast for 2030 says that slice of the pie grows to 20%, but again, big giant asterisk. I'll ballpark the 2016 figure at 10—15%.
I'm really not worried about a slight inconvenience to less than 20% of the users of an interchange if it means a better experience for the majority. And it really is only a slight inconvenience, as the two signals at a DDI operate on simple 2-phase cycles, so the green time at each is nearly half. Also, my perspective on this whole thing is by comparing the current DDI to the conventional diamond interchange it replaced, which had significant delays (especially from EB Roberts to NB I-270 in the afternoon). Other interchange designs may operate comparably well, but would have been significantly more costly to build, and it would be difficult to convince me that cost is justified given how adequately the DDI here operates.
Perhaps it's worth having a discussion about signal progression: its advantages, its requirements, factors that reduce its effectiveness, and situations where it's really not even relevant. But that is a highly technical discussion that belongs in its own thread, because its fine details tend to overwhelm threads that are supposed to be about other topics.
Quote from: vtk on September 18, 2016, 05:11:15 PMAfter thinking a bit more about it, I decided those figures still don't quite illustrate my point as clearly as I'd like.
I believe your point was the DDI at I-270 and Roberts Rd works fine because the amount of through traffic on Roberts Rd is "˜insignificant'. After reading the study you have backtracked on that a little and now describe the Roberts Rd through traffic as "less than 20%" . But if you consider the Roberts Rd traffic that travels through at least one of the DDI signals the percentages exceed 50% (whether you look at 2003 or 2030 volumes).
So far we have only been considering the potential delays on Roberts Road but consider the delay drivers exiting I-270 experience. Every driver exiting the freeway encounters a "NO TURN ON RED" sign regardless of what off-ramp they take. Drivers have to wait for a green light to proceed and are not allowed to make a simple right turn on red in gaps. There are plenty of interchange designs that allow drivers to turn right on red exiting the freeway but this DDI isn't one of them.
Quote from: vtk on September 18, 2016, 05:11:15 PMAlso, my perspective on this whole thing is by comparing the current DDI to the conventional diamond interchange it replaced, which had significant delays (especially from EB Roberts to NB I-270 in the afternoon). Other interchange designs may operate comparably well, but would have been significantly more costly to build, and it would be difficult to convince me that cost is justified given how adequately the DDI here operates.
I could agree with that bold statement if you replaced "comparably well" with "better" . Maybe you are so biased towards DDIs that you won't even admit that other interchange designs would perform better. There's no doubt that the DDI leads to unnecessary driver delays when compared to other interchange designs. Regardless if another interchange would have been practical at I-270 and Roberts Road, you should at least be able to admit that a DDI isn't the most efficient interchange design out there. Anybody with a shred of common sense would be able to admit that.
There are certainly examples of DDIs that permit right turns on red and/or use a yield instead. I'm not sure off-hand why the 270/Roberts DDI isn't one of them (some DOTs do have a general policy not to permit right on red for multi-lane right turns).
Parclos have their own issues, usually involving short acceleration or deceleration distances and merge lanes and unfriendliness to pedestrians and cyclists, in addition to greater land use, so they're certainly not better than a DDI under all circumstances.
Quote from: tradephoric on September 19, 2016, 09:05:34 AM
...Regardless if another interchange would have been practical at I-270 and Roberts Road, you should at least be able to admit that a DDI isn't the most efficient interchange design out there. Anybody with a shred of common sense would be able to admit that.
The most efficient interchange designs are going to be full, non-stop movement designs that don't have any crossing traffic. That's limited to cloverleafs and flyovers. And both of those can have limitations, especially in terms to the room needed, weaving and cost factors (especially overpasses). So then you take a look at the area, including the surrounding area, and figure out which designs will work best for the area based on current and future traffic flows.
Quote from: tradephoric on September 19, 2016, 09:05:34 AM
I could agree with that bold statement if you replaced "comparably well" with "better" . Maybe you are so biased towards DDIs that you won't even admit that other interchange designs would perform better.
No, I meant "comparably well" because other designs have other drawbacks. A SPUI has a three-phase signal instead of the two-phase signals of a DDI, so wait times for traffic turning left to enter the freeway may be longer. A parclo has loops that take more time to drive around than direct ramps. Anything that doesn't fit in the original ROW would probably still not be built yet because of the additional paperwork. In the absence of a perfect solution, there's very little room for improvement over "adequate". Or do you have a perfect solution you would like to share?
Quote from: vtk on September 19, 2016, 10:54:43 PM
A parclo has loops that take more time to drive around than direct ramps.
I think a better argument against Parclos is the sheer amount of redundant land within the loops. DDIs, SPUIs, and regular diamonds are much better fits for urban areas. Even out in the sticks, you still need to buy extra ROW for a Parclo A4 or B4 (though as long as that's not an issue, I will agree with Tradephoric that Parclo B4s are the best interchange you can build, short of full grade-separation).
Quote from: vtk on September 19, 2016, 10:54:43 PM
Quote from: tradephoric on September 19, 2016, 09:05:34 AM
I could agree with that bold statement if you replaced "comparably well" with "better" . Maybe you are so biased towards DDIs that you won't even admit that other interchange designs would perform better.
No, I meant "comparably well" because other designs have other drawbacks. A SPUI has a three-phase signal instead of the two-phase signals of a DDI, so wait times for traffic turning left to enter the freeway may be longer. A parclo has loops that take more time to drive around than direct ramps. Anything that doesn't fit in the original ROW would probably still not be built yet because of the additional paperwork. In the absence of a perfect solution, there's very little room for improvement over "adequate". Or do you have a perfect solution you would like to share?
To determine which interchange has the best operational performance you got to compare common measures of effectiveness. What design has the lowest network delay, lowest average driver stops, and shortest queue distances? You can't answer those questions by simply knowing that it takes more time for drivers to travel around a loop ramp. Your argument lacks any meaningful facts.
Quote from: tradephoric on September 20, 2016, 08:14:41 AM
What design has the lowest network delay, lowest average driver stops, and shortest queue distances? You can't answer those questions by simply knowing that it takes more time for drivers to travel around a loop ramp. Your argument lacks any meaningful facts.
Taking extra time to traverse a loop is network delay, just like waiting at a red light. Besides, your arguments – a lack of "through-roadway experience" for the very small fraction of traffic that might expect it, and a signal that's difficult to coordinate for two-way progression in a corridor that doesn't have two-way signal progression anyway – are not meaningful deficiencies of an interchange at this location.
Do you want to try to find the study in which ODOT evaluated different interchange designs at I-270 & Roberts Rd? I'm sure I saw at least a comparison with different evaluation measures, including something like "operational efficiency", for each design. I'm not saying ODOT concluded the DDI would have the best operational efficiency – though they may have – but they certainly modeled and simulated and analyzed, and didn't just build the thing and hope it would work. I remember seeing concept drawings for a diamond with roundabouts, and maybe a regular diamond with a bunch more lanes, among the alternatives considered. I don't remember whether any parclo designs were evaluated, and I'm pretty sure there was no SPUI alternative at that stage; such designs were likely eliminated early in the process due to cost, and if kept and chosen, likely wouldn't be funded yet.
My whole point is not that DDI interchanges are awesome and never a poor choice. My point is that they can be a sensible choice in some locations, including the one in Columbus. My uninformed assumption that any
other DDI was not a poor choice for that location is based not on bias towards this interchange type, but on a general (perhaps uninformed) assumption of competence within the agencies that build them, paired with the fact that they chose to build a DDI. (I try not to express this as a certainty when talking about interchanges I'm not personally familiar with, hence the "I suspect" phrasing used in a previous post.)
Just as you perceive that I have a bias in favor of DDIs, you are perceived to have a bias in favor of considering signal progression as the single most important factor in roadway & interchange design, with reduced attention to other factors typically considered. Other people, myself included, recognize the benefit of good signal progression, but do not give it enough weight to sink the DDI concept completely. I believe that discussion may be worth having, but not in a thread that's ostensibly about diverging diamond interchanges. Or it may devolve into a religious war, which definitely should be kept separate from a thread that's about anything else.
Quote from: tradephoric on September 19, 2016, 09:05:34 AM
Quote from: vtk on September 18, 2016, 05:11:15 PMAfter thinking a bit more about it, I decided those figures still don't quite illustrate my point as clearly as I'd like.
I believe your point was the DDI at I-270 and Roberts Rd works fine because the amount of through traffic on Roberts Rd is "˜insignificant'. After reading the study you have backtracked on that a little and now describe the Roberts Rd through traffic as "less than 20%" . But if you consider the Roberts Rd traffic that travels through at least one of the DDI signals the percentages exceed 50% (whether you look at 2003 or 2030 volumes).
So far we have only been considering the potential delays on Roberts Road but consider the delay drivers exiting I-270 experience. Every driver exiting the freeway encounters a "NO TURN ON RED" sign regardless of what off-ramp they take. Drivers have to wait for a green light to proceed and are not allowed to make a simple right turn on red in gaps. There are plenty of interchange designs that allow drivers to turn right on red exiting the freeway but this DDI isn't one of them.
Please help me out here, I'm genuinely trying to understand you completely. It has always been my understanding that the selling point of a DDI was to assist with left-turning traffic. That said, at this particular location, the existing bridge deck caused real problems in this regard, as a left turn lane either wouldn't fit, or reduced through traffic to one lane. In addition, although I don't see it mentioned anywhere, a significant part of said traffic is trucks. Now that the DDI is operational, it makes sense to me that a majority of traffic would only encounter one signal. It would seem to me that was the intention of having this particular design at this location...help with the large left turning traffic by having it bypass the second signal.
Before it was built, signal progression was hopeless anyway, since traffic routinely queued past the next set of lights anyway. What am I missing? I feel like there's one piece I'm not catching somewhere.
Here is one of my favorite theoretical interchanges known as the folded interchange:
This interchange would have been completely impractical to build at I-270 and Roberts Road but it would have had better measures of effectiveness than the DDI. The problem I have is when someone suggests that other interchange designs - regardless of how much money they cost - would perform "comparably well" to the DDI. If money was no object they would stop building DDIs entirely (since they are not very efficient). It seems like VTK is trying to backtrack from their "comparably well" statement but just can't do it.
Quote from: tradephoric on September 20, 2016, 09:14:48 PM
Here is one of my favorite theoretical interchanges known as the folded interchange:
This interchange would have been completely impractical to build at I-270 and Roberts Road but it would have had better measures of effectiveness than the DDI. The problem I have is when someone suggests that other interchange designs - regardless of how much money they cost - would perform "comparably well" to the DDI. If money was no object they would stop building DDIs entirely (since they are not very efficient). It seems like VTK is trying to backtrack from their "comparably well" statement but just can't do it.
I see some uncomfortably close calls with RTOR in that video.
Quote from: jeffandnicole on September 20, 2016, 09:44:23 PM
I see some uncomfortably close calls with RTOR in that video.
Also, drivers exiting the loop ramp take it at 70 mph lol. Obviously this is a conceptual model and it took me about 20 minutes to put together. Sometimes drivers do take loop ramps at 70 mph though:
https://www.youtube.com/watch?v=2dvm7-5yMMo
^This scenario is probably one of the strongest arguments against loop ramps, specifically exiting loop ramps. Of course, based on some of the callous comments made on the crash prone roundabout thread regarding fixed objects in the middle of roundabouts, I doubt many people on this forum have a problem with drivers taking loop ramps at 70 mph (since it's their own fault.. personal responsibility... all that good stuff).
Quote from: tradephoric on September 20, 2016, 09:14:48 PM
If money was no object they would stop building DDIs entirely (since they are not very efficient).
I doubt people who own land next to interchanges would be very happy with that.
Quote from: tradephoric on September 21, 2016, 08:53:32 AM
Quote from: jeffandnicole on September 20, 2016, 09:44:23 PM
I see some uncomfortably close calls with RTOR in that video.
Also, drivers exiting the loop ramp take it at 70 mph lol. Obviously this is a conceptual model and it took me about 20 minutes to put together. Sometimes drivers do take loop ramps at 70 mph though:
https://www.youtube.com/watch?v=2dvm7-5yMMo
^This scenario is probably one of the strongest arguments against loop ramps, specifically exiting loop ramps. Of course, based on some of the callous comments made on the crash prone roundabout thread regarding fixed objects in the middle of roundabouts, I doubt many people on this forum have a problem with drivers taking loop ramps at 70 mph (since it’s their own fault.. personal responsibility… all that good stuff).
Many years ago, a woman was killed near my house when she took a diamond interchange ramp at 90 mph and hit a house across the street.
This scenario is probably the strongest argument against straight ramps, especially exiting straight ramps.
Quote from: jeffandnicole on September 21, 2016, 12:51:38 PM
Many years ago, a woman was killed near my house when she took a diamond interchange ramp at 90 mph and hit a house across the street.
This scenario is probably the strongest argument against straight ramps, especially exiting straight ramps.
Well, good point. Fatal crashes can occur at straight diamond ramps too. I'll always remember that Atlanta bus crash on I-75 where the bus driver mistook an exit ramp for a freeway-lane. The bus blew through the bridge deck and landed on it's side on the freeway killing 6.
http://usatoday30.usatoday.com/news/nation/2007-03-02-atlanta-bus-wreck_x.htm
Ok J&N, I've always wondered why agencies seem to favor entering loop ramps onto the freeway as opposed to exiting loop ramps. Agencies must have a reason they favor entering loop ramps. What is it?
Quote from: vdeane on September 21, 2016, 12:43:37 PM
Quote from: tradephoric on September 20, 2016, 09:14:48 PM
If money was no object they would stop building DDIs entirely (since they are not very efficient).
I doubt people who own land next to interchanges would be very happy with that.
VTK argued that other interchange types would perform "comparably well" to the DDI in relation to operational performance. We aren't looking at cost or the environmental impacts of the interchange. The focus was on total network delay. You are muddying the waters by bringing up people crying about owning land next to a freeway?? Thanks for that irrelevant comment.
Quote from: tradephoric on September 21, 2016, 01:14:46 PM
Quote from: jeffandnicole on September 21, 2016, 12:51:38 PM
Many years ago, a woman was killed near my house when she took a diamond interchange ramp at 90 mph and hit a house across the street.
This scenario is probably the strongest argument against straight ramps, especially exiting straight ramps.
Well, good point. Fatal crashes can occur at straight diamond ramps too. I'll always remember that Atlanta bus crash on I-75 where the bus driver mistook an exit ramp for a freeway-lane. The bus blew through the bridge deck and landed on it's side on the freeway killing 6.
http://usatoday30.usatoday.com/news/nation/2007-03-02-atlanta-bus-wreck_x.htm
Ok J&N, I've always wondered why agencies seem to favor entering loop ramps onto the freeway as opposed to exiting loop ramps. Agencies must have a reason they favor entering loop ramps. What is it?
Well, first, that may or may not be true. I've always thought agencies favored diamond interchanges. I guess we would need to see a listing of all the interchanges that exist to see which ones are favored more than others.
If it is true, a few possible reasons:
It requires those on a highway to make fewer decisions. When you have exit options such as A & B, it requires drivers to think which exit do they want. By forcing everyone into one ramp, they can make that decision at a later point. This, personally, is especially true when I see a blue 'Food' or 'Gas' sign on the highway, then realize I needed to concentrate on not only the business I wanted to get to, but also which particular exit I needed.
Those on the highways are going at a faster speed, so they can use a straighter ramp as time to slow it down as they come off the highway.
No left turns are necessary on the two thru roads themselves. Those on the highway exit right at the single interchange. Those on the cross street either exit right and loop down 270 degrees, or exit right and swing right 90 degrees.
In other cases, it can be a preferred method of exiting as per state policy, or even just a particular preference of the engineer or designer.
In most cases, the interchange that was built wasn't the only interchange under consideration. Usually nearly every design option is open for discussion at the beginning of a project; they take a look at the landscape to see what room they have to work with, and then do their analysis to see what works best at each particular location.
Quote from: tradephoric on September 21, 2016, 01:19:33 PM
Quote from: vdeane on September 21, 2016, 12:43:37 PM
Quote from: tradephoric on September 20, 2016, 09:14:48 PM
If money was no object they would stop building DDIs entirely (since they are not very efficient).
I doubt people who own land next to interchanges would be very happy with that.
VTK argued that other interchange types would perform "comparably well" to the DDI in relation to operational performance. We aren't looking at cost or the environmental impacts of the interchange. The focus was on total network delay. You are muddying the waters by bringing up people crying about owning land next to a freeway?? Thanks for that irrelevant comment.
Actually, your focus is on network delay, and only that. In the real world, engineers have a lot more to look at. If an interchange can be built without requiring the taking of land, that is going to be the preferred option. Again, that's real world, not Engineering 201. There are tons of examples where single landowners or groups of people have delayed projects for years, and even cancelled them. Transportation departments just have to work with them as best they can. The optimal design may not ultimately be what is actually constructed.
Quote from: jeffandnicole on September 21, 2016, 01:39:08 PM
Actually, your focus is on network delay, and only that. In the real world, engineers have a lot more to look at. If an interchange can be built without requiring the taking of land, that is going to be the preferred option. Again, that's real world, not Engineering 201. There are tons of examples where single landowners or groups of people have delayed projects for years, and even cancelled them. Transportation departments just have to work with them as best they can. The optimal design may not ultimately be what is actually constructed.
VTK made a very specific statement. You sometimes have trouble following the conversation J&N. This is what VTK said:
Quote from: vtk on September 18, 2016, 05:11:15 PM
Other interchange designs may operate comparably well, but would have been significantly more costly to build
My point is VTK is wrong in suggesting that other interchange designs would operate comparably well to a DDI. If you want to spend a trillion dollars on an interchange it's going to outperform the DDI in every measure of effectiveness imaginable. I gave them a chance to backtrack but they doubled down:
Quote from: vtk on September 19, 2016, 10:54:43 PMQuote from: tradephoric on September 19, 2016, 09:05:34 AM
I could agree with that bold statement if you replaced "comparably well" with "better" . Maybe you are so biased towards DDIs that you won't even admit that other interchange designs would perform better.
No, I meant "comparably well" because other designs have other drawbacks. A SPUI has a three-phase signal instead of the two-phase signals of a DDI, so wait times for traffic turning left to enter the freeway may be longer. A parclo has loops that take more time to drive around than direct ramps. Anything that doesn't fit in the original ROW would probably still not be built yet because of the additional paperwork. In the absence of a perfect solution, there's very little room for improvement over "adequate". Or do you have a perfect solution you would like to share?
It's not about the cost of the interchange. It's about VTK being so biased towards DDI's that they apparently believe a trillion dollar interchange couldn't outperform the DDI (it would only perform "comparably well" to them). Maybe I'm taking VTK's comment too literally, but they got to realize the DDI is not the best interchange design out there. Again, if money was no object we could come up with a more efficient design then the DDI they came up with (and in a smaller footprint to boot).
Quote from: tradephoric on September 21, 2016, 02:14:50 PM
It's not about the cost of the interchange. It's about VTK being so biased towards DDI's that they apparently believe a trillion dollar interchange couldn't outperform the DDI (it would only perform "comparably well" to them). Maybe I'm taking VTK's comment too literally, but they got to realize the DDI is not the best interchange design out there. Again, if money was no object we could come up with a more efficient design then the DDI they came up with (and in a smaller footprint to boot).
Maybe I'm not following the conversation here either, but I think "comparably well" only applies to interchange designs which are comparably practical. If there's no ROW for loops or flyovers, your discussion narrows to only a few interchange designs: SPUIs, diamonds, dumbell or dogbone roundabouts, full roundabouts with two under- or over-passes, and DDIs. "Comparably well" in this case, only applies to interchanges that may have, at one point, been considered practical when replacing the 270/Roberts Rd interchange.
Just looking at a map of the area, I don't think signal progression is important along that stretch of Roberts Road.
Quote from: jakeroot on September 21, 2016, 06:19:47 PM
Maybe I'm not following the conversation here either, but I think "comparably well" only applies to interchange designs which are comparably practical. If there's no ROW for loops or flyovers, your discussion narrows to only a few interchange designs: SPUIs, diamonds, dumbell or dogbone roundabouts, full roundabouts with two under- or over-passes, and DDIs. "Comparably well" in this case, only applies to interchanges that may have, at one point, been considered practical when replacing the 270/Roberts Rd interchange.
I actually think loop ramps would have fit at this interchange. The advantage of the model below is there are zero traffic signals (reducing driver delay) and it has a smaller footprint than the DDI. The disadvantage is extensive bridge work and loop ramps would have been needed to build this interchange.
https://www.youtube.com/watch?v=B8DUMjlKqx8
A similar interchange was built in Florida but it was a Parclo B4 with exiting loop ramps as opposed to entering:
https://www.google.com/maps/@28.34539,-81.5333086,579m/data=!3m1!1e3
Quote from: tradephoric on September 21, 2016, 02:14:50 PM
It's about VTK being so biased towards DDI's that they apparently believe a trillion dollar interchange couldn't outperform the DDI (it would only perform "comparably well" to them).
Again, I'm not "biased toward DDIs". There are at least a few different, practical, interchanges that would work adequately at I-270 & Roberts Rd, considering their advantages and drawbacks, the traffic patterns, and the adjacent intersections. There wouldn't be much difference between the operational efficiencies of the different designs, though each would have its specific disadvantages. I drive through this interchange not infrequently, and at various times of day, and it is my opinion that the general deficiencies of the DDI design are manifest at this specific interchange to a very limited degree – so limited that it's not worth worrying about.
And while I'm not prepared to agree to narrow the conversation to operational efficiency only, with no concern for cost or environmental concerns, I am having a very hard time imagining what kind of interchange could be built here, given unlimited budget and dictatorial power of eminent domain, that would have no operational weaknesses at all, considering the various other crossroads that intersect Roberts Rd. (Cutting off any of them so their traffic has to go a different way is an operational weakness.)
(Don't have time to review tradephoric's most recent post at this time. Will look at it later.)
Quote from: tradephoric on September 21, 2016, 02:14:50 PM
Quote from: jeffandnicole on September 21, 2016, 01:39:08 PM
Actually, your focus is on network delay, and only that. In the real world, engineers have a lot more to look at. If an interchange can be built without requiring the taking of land, that is going to be the preferred option. Again, that's real world, not Engineering 201. There are tons of examples where single landowners or groups of people have delayed projects for years, and even cancelled them. Transportation departments just have to work with them as best they can. The optimal design may not ultimately be what is actually constructed.
VTK made a very specific statement. You sometimes have trouble following the conversation J&N. This is what VTK said:
Quote from: vtk on September 18, 2016, 05:11:15 PM
Other interchange designs may operate comparably well, but would have been significantly more costly to build
Huh? I was responding to what you said, not what he said, especially in reference to "You are muddying the waters by bringing up people crying about owning land next to a freeway??".
Don't worry about me following the conversation. You're having a difficult enough time yourself.
Quote from: tradephoric on September 22, 2016, 08:52:16 AM
Quote from: jakeroot on September 21, 2016, 06:19:47 PM
Maybe I'm not following the conversation here either, but I think "comparably well" only applies to interchange designs which are comparably practical. If there's no ROW for loops or flyovers, your discussion narrows to only a few interchange designs: SPUIs, diamonds, dumbell or dogbone roundabouts, full roundabouts with two under- or over-passes, and DDIs. "Comparably well" in this case, only applies to interchanges that may have, at one point, been considered practical when replacing the 270/Roberts Rd interchange.
I actually think loop ramps would have fit at this interchange. The advantage of the model below is there are zero traffic signals (reducing driver delay) and it has a smaller footprint than the DDI. The disadvantage is extensive bridge work and loop ramps would have been needed to build this interchange.
https://www.youtube.com/watch?v=B8DUMjlKqx8
I can imagine problems with traffic exiting SB I-270 and cutting across the westbound Roberts Rd lanes to turn left onto Westbelt Dr. Note, the as-built left turn lane for Westbelt begins right about where your design has the ramp from SB I-270 joining Roberts Rd. This could be an issue in PM rush hour, as workers from Hilliard head to their second-shift jobs along Westbelt while office workers coming from Downtown head for their subdivision homes west of the railroad yard. I'm not sure if this can be quantified in terms of vehicle-seconds of delay, or just how bad the problem might get, but it's enough that this design is not clearly better than the DDI that was built. Maybe better, but not clearly so.
By the way, your visualization has a lot of vehicles exiting I-270 only to re-enter it in the opposite direction. That may be a thing that happens on occasion in real life, but not several per minute. Perhaps it's not relevant to our disagreement, and you probably already noticed it, but I felt a need to point it out anyway.
^^^
If the adjacent signals led to problems during rush hour we could tap into our trillion dollar budget and build loop ramps to bypass those signals as well. The fact is my trillion dollar interchange would operationally outperform the DDI (I would hope so, it cost a trillion dollars!). It wouldn't be "comparable" it would be "better" . But you can't even admit that.
Quote from: tradephoric on September 22, 2016, 03:21:00 PM
^^^
If the adjacent signals led to problems during rush hour we could tap into our trillion dollar budget and build loop ramps to bypass those signals as well. The fact is my trillion dollar interchange would operationally outperform the DDI (I would hope so, it cost a trillion dollars!). It wouldn't be "comparable" it would be "better" . But you can't even admit that.
I would have to see a specifc design to estimate its operational efficiency. The issue that concerns me is adding loops and braids to the interchange tends to expand its footprint along the surface road(s), likely introducing more weaving issues as the growing design creeps closer to existing intersections on the fringe of the interchange's scope. Then, to solve those weaving issues, the scope of the interchange is expanded to include those intersections, more loops and braiding is added to fix the weaving issues identified, the interchange design has expanded again, and there are new weaving issues to address between the increasingly enormous interchange and the
next nearest intersections. This is why I don't believe a perfect service interchange design exists for an urban location such as this.
Here's a drive-through of the first DDI in Denmark (my home country 💗), in the community of Odense: https://goo.gl/tkKDTt (E20, junction 52).
https://youtu.be/MDYR6Y1HH-g
It has several characteristics of the American designs, as one might imagine. I think the most notable is that the cross-over movements are completely straight-across. On the first page, near the bottom, I posted an example of a DDI in Durban, South Africa. The cross-over movements are very compact, and require a slight turning maneuver through the intersections. This is necessary when trying to fit the DDI in where a previous diamond interchange already existed; when DDI's are built from scratch, however, the cross-over movements typically don't require any "corning".
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi.imgflip.com%2F2egkk5.jpg&hash=ed2a73daecaab5f8bb26fb2d6dce6d5f1b9b9d12)
Am I doing this right?
Quote from: NE2 on July 21, 2018, 05:50:03 PM
http://i.imgflip.com/2egkk5.jpg
Am I doing this right?
The man in flannel needs to be a diamond, and the bird in blue needs to be a DDI.
EDIT: if this is from trade's perspective, flip the women. Red is a DDI, blue is PARCLO B4. Flannel remains a diamond.
Quote from: jakeroot on July 21, 2018, 05:34:44 PM
Here's a drive-through of the first DDI in Denmark ...
It has several characteristics of the American designs, as one might imagine. I think the most notable is that the cross-over movements are completely straight-across.
When we lay DDI out this is always one of the big debates. Some designers (and DOT) prefer the tangent section to extend from stop lines through the intersection while others don't think it's so crucial. It's sort of funny how opinionated engineers get on some topics even if there aren't that many data points on safety records. Have ANY DDI had significant wrong-way problems regardless of tangents and crossing angles? I'm not aware of any.
https://www.google.com/maps/@35.807396,-80.875909,209m/data=!3m1!1e3
Here is another one that seems very "tight", it's in NC. I don't think I've seen any others with such small radii and lack of tangent for the crossover movements (in north America...the French examples are pretty tight), but if anyone has examples I'd be interested to see them.
The other thing that caught my eye on your Danish example is the "extra" intersection just beyond the main crossover. It seems they're treating the off-ramp left as it's own intersection (or maybe timing it all together but giving extra signal heads and stop lines for reassurance). The driver goes through the main crossover at about 1:25. Then at about 1:30 they are at the intersection for the off-ramp. Off the top of my head, I don't know of any locations like this. For the signal timing nerds: this would impact the all-red time required since the size of the intersection would change.
Quote from: johndoe on July 22, 2018, 04:23:23 PM
The other thing that caught my eye on your Danish example is the "extra" intersection just beyond the main crossover. It seems they're treating the off-ramp left as it's own intersection (or maybe timing it all together but giving extra signal heads and stop lines for reassurance). The driver goes through the main crossover at about 1:25. Then at about 1:30 they are at the intersection for the off-ramp. Off the top of my head, I don't know of any locations like this. For the signal timing nerds: this would impact the all-red time required since the size of the intersection would change.
I've actually designed a few DDI's (literally on paper (https://i.imgur.com/nQxBK6n.jpg)) with a second signal for the off-ramp, to reduce the all-red time for the off-ramp. The off-ramp would need a couple extra seconds of red to allow cars to clear the intersection, but if it had its own signal, it could start earlier. Any cars that made it through just barely on the first signal, would end up stopped at the second signal. In theory, the lights for through-traffic would go red at the same time. However, I would be worried about trucks getting trailers stuck in the crossover point, so ideally it would be at least 25 meters from intersection to stop line. Any less than that, and I think one big intersection would be best. My paper design does not follow my suggestion, but I was like 15 when I drew that, so I didn't give the design much consideration.
Or, you could do it the South African-way, and just have both off-ramps be yields (even if two lanes): https://goo.gl/jJ1gc2. Given that the yield is basically a sharp turn, I would be less worried about drivers ignoring the yield. Not sure if anywhere in the US actually has this as a setup. Many even use NTOR signs for the off-ramps, even if single lane, so they can't even act like it. Of the two examples below, only the first permits turns on red (which is cool since it's a double left turn).
Quote from: johndoe on July 22, 2018, 04:23:23 PM
Quote from: jakeroot on July 21, 2018, 05:34:44 PM
Here's a drive-through of the first DDI in Denmark ...
It has several characteristics of the American designs, as one might imagine. I think the most notable is that the cross-over movements are completely straight-across.
When we lay DDI out this is always one of the big debates. Some designers (and DOT) prefer the tangent section to extend from stop lines through the intersection while others don't think it's so crucial. It's sort of funny how opinionated engineers get on some topics even if there aren't that many data points on safety records. Have ANY DDI had significant wrong-way problems regardless of tangents and crossing angles? I'm not aware of any.
https://www.google.com/maps/@35.807396,-80.875909,209m/data=!3m1!1e3
Here is another one that seems very "tight", it's in NC. I don't think I've seen any others with such small radii and lack of tangent for the crossover movements (in north America...the French examples are pretty tight), but if anyone has examples I'd be interested to see them.
Yeah, there doesn't seem to be significant (any?) evidence that one design is better than another, just some guessing on the part of the lead engineers. I can see why they might not want any tangential movements in the crossover, but I don't know of any evidence that suggests its necessary. Traffic is generally moving quite slow, and since the other direction of the crossover is red, and traffic is probably waiting at it, I don't think drivers would be likely to misconstrue that leg of the intersection as being legally traversable anyway. While neither the US nor Canada is generally known for using an abundance of pavement markings, that's definitely not the case at DDI's; nearly all examples that I've seen use a ton of edge extension markings, those giant one-way arrows painted in front of the stop lines, raised markings for additional reflection, and extra signals
and signs. You'd have to be blind to drive the wrong way.
Your example is definitely quite tight! Especially notable is how close the eastbound off-ramp's left turn is to the crossover point. Nevermind the near complete lack of any sort of curve prior to entering the DDI. I honestly prefer this design, as I'm not a fan of big curves (except in my women...). Engineers really seem to put in some big curves prior to DDI's, mostly so drivers enter the crossover point at a sharper angle, but as we just discussed, there doesn't seem to be any evidence of this being necessary, since wrong-way collisions don't seem to be an issue.
Metro Atlanta seems to have quite a few tight DDIs: I-85 @ GA-140 (https://goo.gl/vQKBLv) (first image), and I-85 @ Pleasant Hill Road (https://goo.gl/spKd5B) (just north of the first example; second image). I think the second example is probably the tightest DDI that I've ever seen.
(https://i.imgur.com/S43mEdN.png)
(https://i.imgur.com/6lnuQjP.png)
They're in early stages of construction of a DDI at I-55 and Weber Rd, in Bolingbrook / Romeoville, Illinois
Rochester (well, technically Brighton) has a DDI at I-590 and Winton Road (https://www.google.com/maps/@43.1076397,-77.5751029,231m/data=!3m1!1e3).
It seems to work OK. My main beef is that it resulted in the install of a bunch of overhead assemblies (https://www.google.com/maps/@43.1063453,-77.5764327,3a,60y,18.43h,87.38t/data=!3m6!1e1!3m4!1sjVjlNft_j4EOZhSloo4TJA!2e0!7i13312!8i6656) which are absolutely gargantuan - way bigger than they needed to be.
Quote from: jakeroot on July 22, 2018, 05:00:27 PM
While neither the US nor Canada is generally known for using an abundance of pavement markings, that's definitely not the case at DDI's; nearly all examples that I've seen use a ton of edge extension markings, those giant one-way arrows painted in front of the stop lines, raised markings for additional reflection, and extra signals and signs. You'd have to be blind to drive the wrong way.
Yeah, this is the case in the above DDI as well, and that makes it kind of ugly. Maybe once drivers become more familiar with DDI's, DOT's won't have to be as excessive about "Wrong Way" signs and pavement markings on new installs.
DDI's basically create a scenario in which making a turning movement, in and of itself,
never requires crossing traffic. Therefore, turning movements are always free flowing (or at least free-flowing a greater percentage of the time). This is only optimal when there's a lot of turning traffic, especially left-turning traffic getting on and off the highway, and little through traffic. This basically applies to Winton and I-590, so it's probably a net positive compared to a standard diamond.
It wasn't horribly congested before, and it still isn't, so it was probably a good test location, while simultaneously having desirable traffic patterns for DDI implementation.
Delaware's diverging diamond at DE-1 and DE-72 was given an award recently: https://www.wdel.com/news/video-deldot-s-diamond-gets-an-award/article_e29bac14-8aab-11e8-8687-032cea41d2a5.html
Quote from: jakeroot on July 22, 2018, 05:00:27 PM
I've actually designed a few DDI's (literally on paper (https://i.imgur.com/nQxBK6n.jpg)) with a second signal for the off-ramp, to reduce the all-red time for the off-ramp. The off-ramp would need a couple extra seconds of red to allow cars to clear the intersection, but if it had its own signal, it could start earlier. Any cars that made it through just barely on the first signal, would end up stopped at the second signal. In theory, the lights for through-traffic would go red at the same time. However, I would be worried about trucks getting trailers stuck in the crossover point, so ideally it would be at least 25 meters from intersection to stop line. Any less than that, and I think one big intersection would be best. My paper design does not follow my suggestion, but I was like 15 when I drew that, so I didn't give the design much consideration.
This is a little different but I have noticed some SPUIs are designed with off-ramps that have signal heads for both the off-ramp and the main arterial while others only have signals for the off-ramp. Here's an example of each:
Off-ramp only signal:
https://www.google.com/maps/@43.5669106,-116.1963375,3a,82.8y,40.27h,89.92t/data=!3m6!1e1!3m4!1sCWQYFuuGVobMqmEmpME0SQ!2e0!7i13312!8i6656
Main+Off-ramp signal:
https://www.google.com/maps/@35.9096677,-78.935117,3a,63.7y,23.23h,85.72t/data=!3m6!1e1!3m4!1sbBthmzhZ3GX6YdPlhmKJoA!2e0!7i13312!8i6656
The example of the off-ramp only signal seems like a potential safety issue to me. How does the arterial know to stop when the off-ramp has a green indication when there are no signal heads along the arterial? Now presumably there is enough all red for the left-turn off-ramp traffic to clear the opposing right turn off-ramp with time to spare, but what if the arterial is backed up through the SPUI for some reason? In that case the main arterial traffic has no indication that the right-turn off-ramp has a green.
Quote from: tradephoric on July 23, 2018, 10:16:27 AM
Quote from: jakeroot on July 22, 2018, 05:00:27 PM
I've actually designed a few DDI's (literally on paper (https://i.imgur.com/nQxBK6n.jpg)) with a second signal for the off-ramp, to reduce the all-red time for the off-ramp. The off-ramp would need a couple extra seconds of red to allow cars to clear the intersection, but if it had its own signal, it could start earlier. Any cars that made it through just barely on the first signal, would end up stopped at the second signal. In theory, the lights for through-traffic would go red at the same time. However, I would be worried about trucks getting trailers stuck in the crossover point, so ideally it would be at least 25 meters from intersection to stop line. Any less than that, and I think one big intersection would be best. My paper design does not follow my suggestion, but I was like 15 when I drew that, so I didn't give the design much consideration.
This is a little different but I have noticed some SPUIs are designed with off-ramps that have signal heads for both the off-ramp and the main arterial while others only have signals for the off-ramp. Here's an example of each:
Off-ramp only signal:
https://www.google.com/maps/@43.5669106,-116.1963375,3a,82.8y,40.27h,89.92t/data=!3m6!1e1!3m4!1sCWQYFuuGVobMqmEmpME0SQ!2e0!7i13312!8i6656
Main+Off-ramp signal:
https://www.google.com/maps/@35.9096677,-78.935117,3a,63.7y,23.23h,85.72t/data=!3m6!1e1!3m4!1sbBthmzhZ3GX6YdPlhmKJoA!2e0!7i13312!8i6656
The example of the off-ramp only signal seems like a potential safety issue to me. How does the arterial know to stop when the off-ramp has a green indication when there are no signal heads along the arterial? Now presumably there is enough all red for the left-turn off-ramp traffic to clear the opposing right turn off-ramp with time to spare, but what if the arterial is backed up through the SPUI for some reason? In that case the main arterial traffic has no indication that the right-turn off-ramp has a green.
Sure enough you can run into similar situations at DDIs. Here is a streetview of a DDI in Atlanta.
https://www.google.com/maps/@33.9171717,-84.337966,3a,60y,334.31h,82.55t/data=!3m6!1e1!3m4!1s8CZ0B0Qei_URNcYkyzTGvw!2e0!7i13312!8i6656
Once drivers on the arterial pass the main signal, they have no indication if the off-ramp is green or red. So if the arterial is backed up for whatever reason inching through at 2 or 3 mph, there could be 7 cars lined up between the main signal and off-ramp signal (all assuming they can just keep going since they already passed through their green light at the main... even as the off-ramp signal is displaying a green).
Quote from: tradephoric on July 23, 2018, 10:28:20 AM
Once drivers on the arterial pass the main signal, they have no indication if the off-ramp is green or red. So if the arterial is backed up for whatever reason inching through at 2 or 3 mph, there could be 7 cars lined up between the main signal and off-ramp signal (all assuming they can just keep going since they already passed through their green light at the main... even as the off-ramp signal is displaying a green).
Here's an aerial of that DDI basically showing the scenario (but with only 1 or 2 cars queued between the main signal and off-ramp signal as opposed to 7). That silver car in the aerial (queued up through the DDI behind the red car) would have no idea if the off-ramp turned green. The driver of the silver car is going to want to clear the intersection since they already passed through a green at the main signal. Meanwhile the off-ramp traffic that gets a green indication are going to want to go too.
(https://i.imgur.com/Wt2ILNF.jpg)
Quote from: tradephoric on July 23, 2018, 10:16:27 AM
Quote from: jakeroot on July 22, 2018, 05:00:27 PM
I've actually designed a few DDI's (literally on paper (https://i.imgur.com/nQxBK6n.jpg)) with a second signal for the off-ramp, to reduce the all-red time for the off-ramp. The off-ramp would need a couple extra seconds of red to allow cars to clear the intersection, but if it had its own signal, it could start earlier. Any cars that made it through just barely on the first signal, would end up stopped at the second signal. In theory, the lights for through-traffic would go red at the same time. However, I would be worried about trucks getting trailers stuck in the crossover point, so ideally it would be at least 25 meters from intersection to stop line. Any less than that, and I think one big intersection would be best. My paper design does not follow my suggestion, but I was like 15 when I drew that, so I didn't give the design much consideration.
This is a little different but I have noticed some SPUIs are designed with off-ramps that have signal heads for both the off-ramp and the main arterial while others only have signals for the off-ramp. Here's an example of each:
Off-ramp only signal:
https://www.google.com/maps/@43.5669106,-116.1963375,3a,82.8y,40.27h,89.92t/data=!3m6!1e1!3m4!1sCWQYFuuGVobMqmEmpME0SQ!2e0!7i13312!8i6656
Main+Off-ramp signal:
https://www.google.com/maps/@35.9096677,-78.935117,3a,63.7y,23.23h,85.72t/data=!3m6!1e1!3m4!1sbBthmzhZ3GX6YdPlhmKJoA!2e0!7i13312!8i6656
The example of the off-ramp only signal seems like a potential safety issue to me. How does the arterial know to stop when the off-ramp has a green indication when there are no signal heads along the arterial? Now presumably there is enough all red for the left-turn off-ramp traffic to clear the opposing right turn off-ramp with time to spare, but what if the arterial is backed up through the SPUI for some reason? In that case the main arterial traffic has no indication that the right-turn off-ramp has a green.
I haven't used that first SPUI enough (at least in that direction) to say if there's any problems caused there. I have used this similar one pretty often like a mile away though: https://www.google.com/maps/@43.5717278,-116.2146678,3a,82.9y,287.9h,84.74t/data=!3m6!1e1!3m4!1sOQXxi3nfFiSvwdHe2cbpYw!2e0!7i13312!8i6656
I have never seen any problems here, except 95% of the time I am turning right on red anyways, so hard to say for sure.
But there is another interchange nearby (not a SPUI) where I saw this kind of thing all the time: https://www.google.com/maps/@43.5976763,-116.3543267,3a,75y,276.48h,89.27t/data=!3m6!1e1!3m4!1swHhQb3Of67oxAr3QrQLF3Q!2e0!7i13312!8i6656
Basically, the thru traffic northbound on Eagle Rd would go through the "main" part of the intersection on green, but then would immediately get stopped due to backed up traffic from the St Luke's St light. Then, the thru traffic light would turn red, and the right turning traffic from the off ramp would get the green. Traffic would still be backed up, with cars waiting in the "no man's land" between the crosswalk where the thru green light is, and the curve where the right turners turn on. All this is on a 50 MPH speed limit road lol, driving that area is awful. People always seemed to handle the situation pretty well, though (the right turners were usually good about waiting for traffic to clear before entering).
The most likely SPUI in the area you would run into this issue possibly is this one: https://www.google.com/maps/@43.5931344,-116.3941467,330m/data=!3m1!1e3
It's not on Street View yet. I've used it a handful of times without any issues (but never seen it super backed up or abnormally slow moving).
Going back to your first example, I wonder if they use cameras to see if there are any vehicles coming. Might be a bit advanced, but the technology could exist (ACHD already uses cameras for vehicle detection county-wide), and this
is a brand new SPUI. I don't see any cameras that appear to be pointing in the correct direction to do that, though, so probably not. There are no real bottlenecks on either side of the SPUI though so it's unlikely to run into backed up traffic that would be possible to cause those kind of issues.
To bring this DDI related, I just realized the recently upgraded Gowen Rd/I-84 interchange would have been a really good place to try out a DDI in Boise. Shame they didn't, maybe they had good reasons not to (proximity to Eisenman Rd?). At least the new diamond works pretty well (though I haven't been through during Micron rush hours).
Quote from: tradephoric on July 23, 2018, 10:49:31 AM
That silver car in the aerial (queued up through the DDI behind the red car) would have no idea if the off-ramp turned green. The driver of the silver car is going to want to clear the intersection since they already passed through a green at the main signal. Meanwhile the off-ramp traffic that gets a green indication are going to want to go too.
How is that any different from this situation?
http://www.google.com/maps/@28.3780009,-81.5046222,3a,75y,4.05h,91.91t/data=!3m6!1e1!3m4!1sHAYBhjB_JvlD2O2NQTnLNQ!2e0!7i13312!8i6656
Traffic can back up here from the next light (Hotel Plaza Boulevard). How is traffic stuck just beyond this light to know when the right turn from the ramp on the right turns green? It's not; traffic on the ramp is supposed to wait until the intersection is clear, just like at every other intersection.
Quote from: NE2 on July 23, 2018, 06:35:12 PM
Quote from: tradephoric on July 23, 2018, 10:49:31 AM
That silver car in the aerial (queued up through the DDI behind the red car) would have no idea if the off-ramp turned green. The driver of the silver car is going to want to clear the intersection since they already passed through a green at the main signal. Meanwhile the off-ramp traffic that gets a green indication are going to want to go too.
How is that any different from this situation?
http://www.google.com/maps/@28.3780009,-81.5046222,3a,75y,4.05h,91.91t/data=!3m6!1e1!3m4!1sHAYBhjB_JvlD2O2NQTnLNQ!2e0!7i13312!8i6656
Traffic can back up here from the next light (Hotel Plaza Boulevard). How is traffic stuck just beyond this light to know when the right turn from the ramp on the right turns green? It's not; traffic on the ramp is supposed to wait until the intersection is clear, just like at every other intersection.
I'm not suggesting this problem is only limited to DDIs. The initial example i gave was at a SPUI after all. I just think DDI's (and SPUI's for that matter) have geometries that are more conducive to this type of "no man's land" issue than a typical intersection. At Median U-Turns, where you have large queue space between the medians, there will be both near and far signals to help ensure that drivers crossing the "far side" of the intersection will stop when the opposing side-street is being displayed a green indication. Like this:
https://www.google.com/maps/@42.5626525,-83.1287596,3a,60y,1.98h,88.84t/data=!3m6!1e1!3m4!1sfbB6v3zLzfZ3MES9Q8PlXA!2e0!7i13312!8i6656
Quote from: tradephoric on July 23, 2018, 10:49:31 AM
Quote from: tradephoric on July 23, 2018, 10:28:20 AM
Once drivers on the arterial pass the main signal, they have no indication if the off-ramp is green or red. So if the arterial is backed up for whatever reason inching through at 2 or 3 mph, there could be 7 cars lined up between the main signal and off-ramp signal (all assuming they can just keep going since they already passed through their green light at the main... even as the off-ramp signal is displaying a green).
Here's an aerial of that DDI basically showing the scenario (but with only 1 or 2 cars queued between the main signal and off-ramp signal as opposed to 7). That silver car in the aerial (queued up through the DDI behind the red car) would have no idea if the off-ramp turned green. The driver of the silver car is going to want to clear the intersection since they already passed through a green at the main signal. Meanwhile the off-ramp traffic that gets a green indication are going to want to go too.
(https://i.imgur.com/Wt2ILNF.jpg)
Not a totally unusual situation. I've seen this elsewhere at relatively normal intersections where timing may not allow for full clearance, especially in the event of traffic congestion. Normal people don't intentionally hit each other, so they just let the remaining vehicle go, or merge in as necessary.
I-4 Exit 58 is slated for one soon. With all the left turns that take place there, it will work perfectly once in place. Though not a fan of them, I do commute through here daily and its a big issue with CR 532 turning left onto I-4 and those exiting I-4 WB turning left onto CR 532.
Florida is planning a DDI at I-4 and Martin Luther King Jr. Blvd:
(https://i.imgur.com/VZMrPAF.jpg)
The state hasn't abandoned the Parblo B4 though. This Parclo B4 at I-75 & Sheridan Street was just recently reconstructed:
(https://i.imgur.com/1sa0meb.jpg)
It looks like a Parclo B4 would fit in the footprint of the proposed DDI at I-4 & MLK Blvd. At this point I'm just glad that Florida didn't replace the Parclo B4 at I-75 & Sheridan Street with a DDI.
Florida likes to go big with their DDIs. Here's the first one they built along University Parkway:
(https://i.imgur.com/i7SdNlm.jpg)
All that room....they could at least put in a shared use path. And that applies for all three diagrams above this post. I love big interchanges as much as the next AARoads user, but they are just not friendly to the neighborhood at all. And agencies really don't seem to do enough to rectify this situation.
What really bothers me is when states put in a DDI to, maybe, give the appearance that they are ped-friendly, but they are perhaps less friendly than even a large Parclo interchange.
I would really like states to start installing grade-separated shared use paths through interchanges. In the Parclo interchange above (75 @ Sheridan), a wide (20ft+) shared use path could have been built below the slip ramps (and then above the freeway), making the two sides of the freeway much better connected. The sidewalks that have been used look to be painfully narrow. I can't imagine the noise!
In that Sheridan interchange, I also noticed what is a big no-no in my area: crosswalks divided into two crossing sections. The intersection where the loops meet Sheridan have two pedestrian heads (https://goo.gl/2QDsSP) installed for each slip ramp crossing. This is fine if they are synced together (as in, no need to push a second button mid-way across), but if they are divided for purposes of improving minimum signal phasing time? Not cool.
O.o
Grade-separated shared-use paths? Through intersections?
Totally infeasible due to the benefits gained from the immense costs.
Quote from: Rothman on July 25, 2018, 02:33:07 PM
O.o
Grade-separated shared-use paths? Through intersections?
Totally infeasible due to the benefits gained from the immense costs.
*Interchanges. I don't expect it at every intersection.
"Immense costs"? It's not much compared to the rest of the interchange.
Note this interchange below. I don't know where it's from, but it has a third grade for peds. Brilliant!
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fwww.saveoakhill.org%2Fwp-content%2Fuploads%2F2015%2F07%2Froundabout2.jpg&hash=a075eacbc515b53d2300efc794b910b281084262)
Even through interchanges it is an incredible expense. Hope away, but it will never be standard practice.
Quote from: Rothman on July 25, 2018, 02:49:06 PM
Even through interchanges it is an incredible expense. Hope away, but it will never be standard practice.
I don't expect them to be used in the middle of nowhere, but any time you have a freeway running through an urban or suburban area, agencies need to do their part to keep all network types tied together. This means cars, transit, bikes, and pedestrians. All are accounted for in all modern interchanges, but the first two (primarily the first) clearly gets top priority. Why? Because we do so little to accommodate for pedestrians, that nobody wants to walk anywhere. That leads to less pedestrians, more drivers, etc. If that's our end goal, fine. Just build the minimum requirements. But if we want less cars, we need to make it less of a pain to get around on foot. Florida sucks to walk around in during the summer months, but the rest of the year, it's quite pleasant!
By the way, it's becoming more common in the Netherlands, even in suburban areas.
Quote from: jakeroot on July 25, 2018, 02:30:25 PM
What really bothers me is when states put in a DDI to, maybe, give the appearance that they are ped-friendly, but they are perhaps less friendly than even a large Parclo interchange.
Yeah, pedestrians along the University Parkway DDI have to cross 14 lanes of traffic — and 10 of those lanes cross the main-arterial. The pedestrian exposure is worse at the University Parkway DDI compared to the Parclo example (pedestrians at the Sheridan Parclo only cross 11 lanes of traffic — and none of those lanes cross the main-arterial). There does seem to be a disconnect to what engineers view as "pedestrian friendly" . Charles Marohn, who runs Strong Towns, posted a tour of the "pedestrian friendly" diverging diamond rebuttal video which is pretty good. The original video featured an overly enthusiastic engineer talking about a newly constructed "pedestrian friendly" DDI.
https://www.youtube.com/watch?v=zWG49xlZ_eQ
Quote from: tradephoric on July 25, 2018, 03:41:54 PM
Quote from: jakeroot on July 25, 2018, 02:30:25 PM
What really bothers me is when states put in a DDI to, maybe, give the appearance that they are ped-friendly, but they are perhaps less friendly than even a large Parclo interchange.
Yeah, pedestrians along the University Parkway DDI have to cross 14 lanes of traffic — and 10 of those lanes cross the main-arterial. The pedestrian exposure is worse at the University Parkway DDI compared to the Parclo example (pedestrians at the Sheridan Parclo only cross 11 lanes of traffic — and none of those lanes cross the main-arterial). There does seem to be a disconnect to what engineers view as "pedestrian friendly" . Charles Marohn, who runs Strong Towns, posted a tour of the "pedestrian friendly" diverging diamond rebuttal video which is pretty good. The original video featured an overly enthusiastic engineer talking about a newly constructed "pedestrian friendly" DDI.
https://www.youtube.com/watch?v=zWG49xlZ_eQ
He definitely summed up a lot of my issues with many DDIs. Chiefly, the requirement to cross into the middle of the interchange to get across. AFAIK, this is because old bridges are utilized in the conversion, and the crossover requires the roadways be pushed to edge of the bridge, displacing the crosswalk to the middle.
Of course, that's an excuse. A properly designed interchange should consider all users equally, even if certain users aren't in the majority. You can design a high capacity interchange with excellent pedestrian and cycling facilities (the roundabout above is just one example), but most engineers seem to like using the whole budget to improve things for cars, leaving barely any of the budget for other modes of access.
Folded diamonds are actually really nice for pedestrians, because they limit the number of crossings to two T-intersections. That may be why they are so popular in the Netherlands. You could even add in an extra slip ramp for traffic, so that the loops would only be used for one direction of traffic, and you'd still only have two roadways to cross. Hell of a lot better than the DDI.
The only proper DDI design, IMO, are the American Fork DDI's (https://goo.gl/23Ptp7) in Utah. New bridges were built, and pedestrian access was kept to the exterior of the roadways. Four lanes to cross at the lower DDI, six at the upper. At least double that if the sidewalk were in the inside. I am staggered that the University Pkwy DDI wasn't designed this way, since it was built from the ground-up.
Quote from: jakeroot on July 25, 2018, 04:18:17 PM
The only proper DDI design, IMO, are the American Fork DDI's (https://goo.gl/23Ptp7) in Utah. New bridges were built, and pedestrian access was kept to the exterior of the roadways. Four lanes to cross at the lower DDI, six at the upper. At least double that if the sidewalk were in the inside.
Nah. Most built now have the pedestrians in the middle, and for good reason. Here's an excerpt from Utah's DDI guide:
"UDOT prefers that pedestrians cross DDIs using an island
located in the middle of the interchange. While the first
of Utah's DDIs crossed pedestrians on the outside of the
interchange, this placed them in the position of crossing
free movements at locations with limited visibility. Recent
designs have shifted pedestrians to the median. This improves
lines of sight for both pedestrians and vehicles to
and from the crossings and allows pedestrians to cross
through traffic at a signalized location to clarify pedestrian/vehicle
rights of way. Left turns to the entrance ramps
can also run freely without conflict with pedestrian crossings."
While you guys are right that pedestrians cross more lanes, they're slower lanes coming to a signal. Even in the case of University Pkwy with ~100' to cross, you'll get plenty of time since the crossing is tied to one of the "through" phases (which will get enough green time). It's not like other designs that would be thrown out of coordination for many cycles if a pedestrian crossed the road.
So signalize the turns to and from the ramps. Dumb reasoning.
Alternatively, design the crosswalks so that pedestrians are more visible.
Quote from: johndoe on July 25, 2018, 10:33:52 PM
While you guys are right that pedestrians cross more lanes, they're slower lanes coming to a signal. Even in the case of University Pkwy with ~100' to cross, you'll get plenty of time since the crossing is tied to one of the "through" phases (which will get enough green time). It's not like other designs that would be thrown out of coordination for many cycles if a pedestrian crossed the road.
You're not getting it. All modern interchanges assist pedestrians getting to and from each side. That's not the issue here. The consideration needs to be: do pedestrians want to cross here? What can we do to make this area less frightening? Not just "does this work?". The American Fork DDIs allow pedestrians to interact with the roadways as little as possible, making them much more inviting for pedestrians.
FWIW, it's because of signal timing requirements that I prefer for crossings to be grade separated where feasible. The 75 & Sheridan interchange above is an example of a node where the roadway plans could have been scaled back slightly to allow for an exclusive, grade-separated pedestrian path. Remember why everyone drives: because no one wants to walk. Cars are their own worst enemy.
Quote from: jakeroot on July 25, 2018, 11:13:51 PM
So signalize the turns to and from the ramps. Dumb reasoning.
Alternatively, design the crosswalks so that pedestrians are more visible.
You're not getting it. All modern interchanges assist pedestrians getting to and from each side. That's not the issue here. The consideration needs to be: do pedestrians want to cross here? What can we do to make this area less frightening? Not just "does this work?". The American Fork DDIs allow pedestrians to interact with the roadways as little as possible, making them much more inviting for pedestrians.
FWIW, it's because of signal timing requirements that I prefer for crossings to be grade separated where feasible. The 75 & Sheridan interchange above is an example of a node where the roadway plans could have been scaled back slightly to allow for an exclusive, grade-separated pedestrian path. Remember why everyone drives: because no one wants to walk. Cars are their own worst enemy.
Haha I don't want to be a jerk, but I do this work as my career, I think I "get it". I love this forum because I'm a roadgeek too ... but it's a lot different to "design things on paper" with a general interest than it is to lay in vertical and horizontal curves, design the signing/striping, and have your professional signature on a plan set. One of us has done those things...I think my opinion is at least worth hearing. If nothing else people can know why engineers do some of the things we do.
Regarding grade separations... Sure that'd be nice and if there were enough pedestrians at a particular spot (or money) you'd probably see it more often. Just as Rothman said, don't hold your breath. Bridges are expensive, and most decision-makers care about the majority of users (and interchanges could have at least 500x as many cars as peds). I don't always agree, but that's the world we live in. Also keep in mind crossing an interchange is probably no worse than the signals all the way down the corridor. You'd probably need bridges at those, too. Also keep in mind that really long pedestrian structures have a reputation (haven't seen data) for crime. And have fun with the ADA requirements.
On convincing people to walk, absolutely that FL DDI looks daunting. But take a look at both signals adjacent to it. Long crossings and a lack of refuge islands mean the pedestrian simultaneously crosses vehicles going different directions. With the DDI you cross one direction of traffic at a time. You'll get plenty of time without messing up signal timing for cars. It also gives you the chance to cross from north to south side. That's something many interchanges don't do.
As far as designing the crossings on outside of DDI for visibility... Good luck. Especially if the freeway goes over (many bridge piers and shadows hiding peds). Again this is why I posted the Utah guidance... I think they know what they're talking about :bigass:
As far as signalizing the lefts onto the freeway...that completely removes one of the benefits of the DDI (free lefts) and adds the complexity of an additional signal. Avoiding this is certainly not "dumb reasoning". It's also tough to place the signal heads in "crossroad under" situations. This has been built though, I think Missouri has a couple like that.
Again I don't want to be condescending, but hopefully this describes why peds in middle is (and will be) more common.
Speaking of pedestrian access to a DDI, here's the preferred alignment document for the Weber Rd. I-55 interchange...
http://www.idot.illinois.gov/Assets/uploads/IDOT-Projects/District-1/I55-at-Weber-Rd-Study/files/3_Preferred%20Alternative.pdf
http://www.idot.illinois.gov/Assets/uploads/IDOT-Projects/District-1/I55-at-Weber-Rd-Study/files/6_Aerial%20Interchange.pdf
This is going to be a huge project as it includes a couple miles of Weber Rd to the south, and portions of Normantown Rd.
Full project site:
http://www.idot.illinois.gov/projects/i55-at-weber-road
From my understanding DDI's often don't coordinate to adjacent signals because they need to run shorter cycles to prevent on/off ramp traffic from queuing up between DDI signals. But in the Florida example, the massive capacity of that University Parkway DDI has me questioning if they are attempting to coordinate with the adjacent signals. The DDI at University Parkway isn't the first DDI built along a 6-lane arterial yet it is by far the highest capacity DDI built in America to date. IMHO, if a 6-lane arterial needs to widen out to 12-lanes at the interchange to achieve an acceptable level of service, there is something wrong with that design (sure, it may move vehicles through it... maybe even efficiently... but football fields worth of asphalt shouldn't be needed to achieve that goal).
Quote from: johndoe on July 26, 2018, 07:34:43 AMAs far as signalizing the lefts onto the freeway...that completely removes one of the benefits of the DDI (free lefts) and adds the complexity of an additional signal. Avoiding this is certainly not "dumb reasoning". It's also tough to place the signal heads in "crossroad under" situations. This has been built though, I think Missouri has a couple like that.
I agree it's not ideal signalizing the lefts onto the freeway. But the alternative is to direct pedestrians to the center median and force them to cross main-arterial traffic (in the case of the University Parkway pedestrians must cross 10-lanes of arterial traffic). The fact that the typical DDI design favors moving traffic efficiently onto the freeway over reducing pedestrian exposure kind of proves that the DDI is not really designed to be pedestrian friendly (which isn't completely surprising considering how few pedestrians are going to cross a DDI to begin with). I'm not expecting a suburban freeway interchange to be pedestrian friendly, but when you have that video of an overly enthusiastic engineer talking about how "pedestrian friendly" the DDI is... there just seems to be a disconnect there. The tick boxes may have been checked to adhere to ADA standards, but many DDI's can still be miserable places to walk (even worse than other suburban freeway interchanges). And even after pedestrians are redirected to the center median and forced to cross main arterial traffic, you still have unsignalized crosswalks where drivers are turning right onto the freeway (and this seems to be the norm at most DDI's).
(https://i.imgur.com/i7SdNlm.jpg)
Quote from: johndoe on July 26, 2018, 07:34:43 AM
Regarding grade separations... Sure that'd be nice and if there were enough pedestrians at a particular spot (or money) you'd probably see it more often. Just as Rothman said, don't hold your breath. Bridges are expensive, and most decision-makers care about the majority of users (and interchanges could have at least 500x as many cars as peds). I don't always agree, but that's the world we live in. Also keep in mind crossing an interchange is probably no worse than the signals all the way down the corridor. You'd probably need bridges at those, too. Also keep in mind that really long pedestrian structures have a reputation (haven't seen data) for crime. And have fun with the ADA requirements.
It's a fact of life because we've designed our roads to be the way they are, and facilitate movements the way they do. Today's engineers are brilliant. There's no denying that. They come up with some very clever stuff, but it's all invariably aimed at improving traffic flow, and
then assisting other users through the junction. I just wish that engineers designed infrastructure with priority from most vulnerable to least vulnerable. You can still have a high capacity junction that is enjoyable for pedestrians to use; the junction just needs to be redesigned from a different perspective. The Dutch are very good at this sort of thing. I'll be damned if the constant protected signals wouldn't drive me crazy as a driver, but I can't fault them -- it works, because it's well designed (unlike protected turns here, which seem to be placed nonsensically).
I understand that "vulnerable" is one of those words that make engineers cringe, but it's a term that's here to stay because it promotes the safe design of an area for users without crumple zones.
All pedestrian corridors can have crime issues, if they aren't well-trafficked. Which is why we need to do more to improve overall pedestrian infrastructure. If we get more people to use the ped paths, there's a far smaller chance that someone will be the victim of a crime. The alternative is to get rid of all pedestrian infrastructure, reducing the chance of ped-on-ped crime to zero since you're not allowed to walk. Though that's a little ridiculous.
Quote from: johndoe on July 26, 2018, 07:34:43 AM
On convincing people to walk, absolutely that FL DDI looks daunting. But take a look at both signals adjacent to it. Long crossings and a lack of refuge islands mean the pedestrian simultaneously crosses vehicles going different directions. With the DDI you cross one direction of traffic at a time. You'll get plenty of time without messing up signal timing for cars. It also gives you the chance to cross from north to south side. That's something many interchanges don't do.
So, the DDI above is acceptable because it's marginally better than the garbage intersections just outside of its perimeter? Good lord, we need to reconsider what we consider "good design".
I can't immediately think of ways to improve the intersections themselves (a lot less room to work with), but we can start at junctions with lots of ROW to at least prove what's possible.
Quote from: johndoe on July 26, 2018, 07:34:43 AM
As far as designing the crossings on outside of DDI for visibility... Good luck. Especially if the freeway goes over (many bridge piers and shadows hiding peds). Again this is why I posted the Utah guidance... I think they know what they're talking about
I don't doubt that they know what they're talking about, but I get the feeling that their infrastructure (like so many agencies out there) is designed for cars first, everyone else second. So the question they're asking becomes, "how can we best help peds through this intersection?" In my opinion, that's the wrong question. What might a better question be? My professors would have to help me there, but one might be, "first, how can we best facilitate vulnerable movements across this freeway?"
For underpasses, the key is for them to be wide (maximum light), and well lit. Thusly, the support piers will have to be designed with a wide ROW in mind.
Quote from: johndoe on July 26, 2018, 07:34:43 AM
As far as signalizing the lefts onto the freeway...that completely removes one of the benefits of the DDI (free lefts) and adds the complexity of an additional signal. Avoiding this is certainly not "dumb reasoning". It's also tough to place the signal heads in "crossroad under" situations. This has been built though, I think Missouri has a couple like that.
To be honest, I'm not particularly worried about a 15-second pedestrian interval at a left turn. It's a minor annoyance for drivers, meanwhile massively improving things for pedestrians and cyclists. If need-be, the on-ramp signals can be tied to the off-ramp signals. The on-ramp would be red at the beginning of the off-ramp phase, and then go green after the crossing period finished. Since off-ramp traffic is not getting back on, the demand for the left turn would be minimal during this time. It might be wise to design DDI's with less option-lane left turns if this were to become more popular.
Quote from: tradephoric on July 26, 2018, 01:19:02 PM
IMHO, if a 6-lane arterial needs to widen out to 12-lanes at the interchange to achieve an acceptable level of service, there is something wrong with that design (sure, it may move vehicles through it... maybe even efficiently... but football fields worth of asphalt shouldn't be needed to achieve that goal).
I agree 100%. So, in summary, the DDI is a terrible experience for pedestrians, and needs 12+ lanes of traffic to work correctly. I wouldn't call that a pass.
Coming back to this after a day and a half, I realize that I might have gone off the rails just a bit. I am still not a fan of central crossings at DDI's, and I think it's wise to consider designing corridors that pedestrians would want to use, not just what they can use. But, given how much more geometric restriction there is on roadway design, agencies really have no choice but to design the roads first, and then work in a pedestrian corridor from there. It should then be up to the agency to properly redesign the roadway to best accommodate the pedestrian corridor.
For example, re-aligning slip lanes so that they point at the roadway closer to a 70- or 80-degree angle, to improve pedestrian visibility. Or, installing a signalized crossing where a yield crossing currently exists. Small changes that don't necessarily hurt vehicle access, but make areas far more inviting to pedestrians.
After a week of solid reading about public spaces, it's amazing how picky humans are about our space. In a car, we seem to be a bit more forgiving. But on foot, we notice everything, and very quickly realize an area's problems.
Quote from: jakeroot on July 28, 2018, 04:26:48 AM
But, given how much more geometric restriction there is on roadway design, agencies really have no choice but to design the roads first, and then work in a pedestrian corridor from there.
I think you're getting into the heart of the issue there. Motorist level of service trumps all other factors in street design in the US, often at the expense of delaying other road users by minutes or making certain trips effectively impossible without a car.
If you listened to some traffic engineers (especially the oldtimers), you'd think the earth would implode if we reduced traffic lanes on city streets from 11'—12' to 9'—10', or prohibited RTOR by default in urban areas.
But basic experience can confirm how ridiculous that is–the Bronx River Parkway, for example, frequently squeezes down to 8 (!) feet per lane in some places, and passenger cars manage to rip 70 down that road. Have you ever looked at a street and gone, "damn, these 10-foot lanes are really delaying the bus and truck traffic on this road"? Of course not, because buses and trucks manage 10-foot lanes fine on plenty of city streets.
NYC and European cities ban RTOR by default and there aren't protests in the street about it like engineers act like there would be, because people realize the delay to motorists is in exchange for significantly lower pedestrian crash rates.
There's no reason why our most pressing concern should be saving seconds for motorists when the same decisions made to that end cause minutes of inconvenience to other users.
This seems like the general thread for DDIs from a quick search, but if there's a better thread, let me know.
Found a DDI (that's supposedly temporary) in the Netherlands, at A44 and N206 (https://www.google.com/maps/@52.1657433,4.4516624,328m/data=!3m1!1e3) at Leiden. The configuration itself is interesting, with two right turn lanes from WB to NB, one from a frontage road and one from the through road. It seems like the final (and permanent) configuration for this interchange is a B4 parclo, from recent Google Maps markings.
(https://i.imgur.com/0EDvmL0.png)
Is a diverging diamond interchange better than a dumbbell or dogbone?
Quote from: tolbs17 on November 03, 2021, 06:37:22 AM
Is a diverging diamond interchange better than a dumbbell or dogbone?
Yes, but I don't think they have similar use cases. In most of America, drivers can't use multi-lane roundabouts correctly, so a dumbbell or dogbone would only be helpful for 2-lane (1 lane each direction) roads. DDIs are mostly for multi-lane roads.
Quote from: tolbs17 on November 03, 2021, 06:37:22 AM
Is a diverging diamond interchange better than a dumbbell or dogbone?
Totally dependent on the situation. DDIs are good where a lot of traffic enters or exits the highway. If the traffic is mainly heavy thru traffic, DDIs can cause significant congestion.
Not sure if it has already been mentioned, but Oklahoma's first DDI is open!
Quote from: Rothman on July 25, 2018, 02:33:07 PM
O.o
Grade-separated shared-use paths? Through intersections?
Totally infeasible due to the benefits gained from the immense costs.
Responding a few years late here, but MnDOT says hi (https://www.google.com/maps/@46.8470459,-96.7684226,494m/data=!3m1!1e3)...
Does it work in every situation? No. But it also isn't the budget buster you claim it to be. In this case, all it took was a little extra grading and a couple of oversized culverts.
I just know that there is not a single diverging diamond interchange in New Jersey. And none are considered there.
They are pretty rare in Ohio, There's a decent amount in Missouri and a lot more being considered here in North Carolina.
Quote from: Scott5114 on March 22, 2009, 04:07:24 AM
The DDI seems to me to be a solution in search of a problem. Having
two "cross-over" locations is just asking for trouble...you're bound
to get Grandpa Artie all confused and driving into incoming traffic.
I'm not sure if the additional thought required of drivers is too much
for the safety improvement of not having to turn left. It is inferior
to the SPUI because it could still require waits at two signals,
whereas the SPUI requires only one (though left-turning traffic from
the ramps will only have to have one, they will have a yield to
contend with, which can potentially be as long of a wait as a
stoplight if traffic is heavy). About the only good thing about the
DDI is that it's cheaper than a SPUI.
Nineteen-year-old me was an idiot.
Quote from: Scott5114 on March 22, 2009, 04:07:24 AM
The DDI seems to me to be a solution in search of a problem. Having two "cross-over" locations is just asking for trouble...you're bound to get Grandpa Artie all confused and driving into incoming traffic.
<snipped>
Quote from: Scott5114 on December 16, 2021, 07:35:15 PM
Nineteen-year-old me was an idiot.
Actually, one of the reasons that an elderly relative quit driving was that the new DDI here was too confusing. We are glad that she agreed before she tried it out.
Obviously, an elderly person can experience enough cognitive decline that driving in any sort of situation becomes inadvisable...but as Road Guy Rob points out in his video on DDIs, wrong-way crashes are comparatively rare in DDIs, because they're designed in such a way that driving into incoming traffic simply doesn't feel like the correct thing to do.
I still prefer SPUIs over DDIs, but I now feel like a DDI is often a preferable solution to a traditional diamond.
I like DDIs mostly because GDOT actually builds them. Before DDIs became popular, there were several congested service interchanges in Metro Atlanta and the only thing GDOT would ever do is study alternatives and not build anything, but when they found out about DDIs, they end up actually building it, possibly because of how much $ is saved by not replacing the existing bridges.
SPUIs are nice and Metro Atlanta has several, but GDOT hasn't built a new one in a long time.
Quote from: ran4sh on December 16, 2021, 11:42:40 PM
SPUIs are nice and Metro Atlanta has several, but GDOT hasn't built a new one in a long time.
Does that include the highway going over the arterial? Because NCDOT has still built SPUIs like that on I-840 in Greensboro and I-295 in Fayetteville.
Newsflash. Tonight (Feb 23, 2022) in its 6PM news hour, WRAL-5 in Raleigh is doing an expose on the safety issues related to the new Diverging Diamond Interchanges (DDIs) recently installed in the Triangle. I am curious if their findings are any different than what has already been discussed here on AARoads.
Quote from: Dirt Roads on February 23, 2022, 11:31:06 AM
Newsflash. Tonight (Feb 23, 2022) in its 6PM news hour, WRAL-5 in Raleigh is doing an expose on the safety issues related to the new Diverging Diamond Interchanges (DDIs) recently installed in the Triangle. I am curious if their findings are any different than what has already been discussed here on AARoads.
So that refers to the one built here (https://www.google.com/maps/@35.7841809,-78.6996656,15.67z). I still think it's safer compared to the previous design which was a death trap. If they are making complaints about it, then I might see it going to just a simple diamond or a Parclo which has 6 ramps.