Are diverging diamonds a fad?

[vdeane]

If one is passing through the DDI entirely on the surface road, you should only get at most one of the lights red, not both.  If you hit two red lights traversing the DDI, at least if it's timed like the one in Rochester, it means you turned left off the freeway, lost a race with a snail, or ran the red.  In any case, going through a DDI at the speed limit is impossible anyways due to the curves at the crossovers (unless the speed limit is already 25).

NYSDOT has "no turn on red" signs on the left turns of its DDI, so I presume that a left on red would have been legal were it not for the sign.

[mrsman]

One thing that always bothered me about the DDI is that two opposing directions of traffic must cross each other.  To me, this is almost like split-phasing, where eastbound and westbound traffic have completely separate phases - thereby increasing delays for both directions.  So yes a DDI really should only be used if the most important movement is surface street to freeway.  Otherwise, use a different design.

I grew up in California, where as many here know, they extensively use the parclo A4.  It seems that this is preferred to eliminate a situation where there are so many cars wishing to make a left turn onto the on-ramp, that it would block the left lane of through traffic on the arterial.  (From what I remember this was the reason for converting some diamonds into SPUIs).  You see this at many diamond interchanges.  To me, it would seem that a parclo B4 could potentially have his problem as well.  So if the left turning movement from surface street to freeway is particularly heavy, maybe a parclo A4 is the answer, otherwise use the parclo B4, as it would reduce signal delay on the arterial.

[tradephoric]

If one is passing through the DDI entirely on the surface road, you should only get at most one of the lights red, not both.

Adjacent signals often run higher cycle lengths to the DDI signals making it a crapshoot if you get stuck at the first DDI light.  The video below shows green to red signal progression between the two DDI traffic signals along Bangerter Hwy in Utah.  In this Utah example, it's common for drivers to experience back to back red lights while driving the main corridor.

UDOT 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.

https://www.youtube.com/watch?v=RhazZu4xMeA

In any case, going through a DDI at the speed limit is impossible anyways due to the curves at the crossovers (unless the speed limit is already 25).

A driver cruising at 50 mph on Bangerter Hwy is forced to slow down to 25 mph to navigate the DDI.  Slowing down traffic to 25 mph can be a good thing in downtown Manhttan, it's not great along Bangerter Hwy in Utah. 

[tradephoric]

I grew up in California, where as many here know, they extensively use the parclo A4.  It seems that this is preferred to eliminate a situation where there are so many cars wishing to make a left turn onto the on-ramp, that it would block the left lane of through traffic on the arterial.  (From what I remember this was the reason for converting some diamonds into SPUIs).  You see this at many diamond interchanges.  To me, it would seem that a parclo B4 could potentially have his problem as well.  So if the left turning movement from surface street to freeway is particularly heavy, maybe a parclo A4 is the answer, otherwise use the parclo B4, as it would reduce signal delay on the arterial.

Wouldn't a SPUI experience the same situation where there are so many cars wishing to make a left turn onto the on-ramp, that it would block the left lane of through traffic on the arterial?  Drivers entering the freeway from a SPUI do so by making a left turn as well.

Also, permissve-protected left-turns at a SPUI are unheard of due to the complexity of the interchange.  There are examples of permissive-protected left-turns at Parclo B4s which help reduce driver delays (and reduce the queue length of left-turning vehicles from the arterial, since some drivers will turn in gaps).  Here's an example of a FYA at a Parclo B4 at I-69 & Saginaw Hwy outside Lansing, Michigan:

https://www.youtube.com/watch?v=doeEKXAoo_0

[mrsman]

I grew up in California, where as many here know, they extensively use the parclo A4.  It seems that this is preferred to eliminate a situation where there are so many cars wishing to make a left turn onto the on-ramp, that it would block the left lane of through traffic on the arterial.  (From what I remember this was the reason for converting some diamonds into SPUIs).  You see this at many diamond interchanges.  To me, it would seem that a parclo B4 could potentially have his problem as well.  So if the left turning movement from surface street to freeway is particularly heavy, maybe a parclo A4 is the answer, otherwise use the parclo B4, as it would reduce signal delay on the arterial.

Wouldn't a SPUI experience the same situation where there are so many cars wishing to make a left turn onto the on-ramp, that it would block the left lane of through traffic on the arterial?  Drivers entering the freeway from a SPUI do so by making a left turn as well.

I was thinking of a situation with extremely short left turn storage lanes.  Basically, the road is wide enough for only one left turn lane, not two. 

Check out this situation on Broadway, near Downtown LA. 

http://goo.gl/maps/S024x

Here, as you scroll, 17th is the "service road" for I-10 west and 18th is the service road for I-10 east.  The left turn pocket for either 18th or 17th is no more than half a block long.  So if there are more people turning left than can fit in the turn pocket, the left thru lane will be blocked.  The length of the left turn pocket is limited by the need to provide for the left turn for the other direction.  Now, if this were somehow turned into a SPUI, the left turn lane from Broadway to 18th could begin even north of 17th, as 18th and 17th would become "single point".  But with a traditional diamond, once the capacity of the turn pocket is reached, the left through lane will be blocked.

Of course, if the surface street were wider, we can have multiple turn lanes to alleviate the problem.

I am sure, though, that for every parclo B4, the left turn lanes are long and/or wide enough to maintain the capacity necessary for the freeway entrance.  That is probably part of the design.  But if is too expensive to widen the arterial's bridge over the freeway (or the arterial's underpass under the freeway), I think parclo A4 is the way to go - since there are no left turns at all.

[jakeroot]

There is a Parclo A4 along the TCH outside Vancouver where the signal only stops one direction, and has a left-side merge with the other carriageway.

[mrsman]

There is a Parclo A4 along the TCH outside Vancouver where the signal only stops one direction, and has a left-side merge with the other carriageway.

There is another Parclo A4 in Maryland (I-95 @ MD 175) where there are left side merges in both directions.  The two carriageways of the cross-street are widely separated though.

https://www.google.com/maps/@39.1775854,-76.7921088,16z/data=!3m1!1e3

[Atomica]

I have noticed the growth of the use of the DDI throughout Missouri myself as I have seen them built in numerous locations - Springfield is laden with them now, and even Joplin has one now at Range Line Road and the I-44 - I don't know that they are a fad.  I do note that we are about to get another interchange along the I-44 with roundabouts on each side of the bridge at Prigmor Road two miles east of the I-49/MO249 junction - a junction also present at Longview Road and the I-49 in Kansas City.

[tradephoric]

The SPUI below demonstrates two points:

1.  Adjacent intersections can limit the amount of left-turn queuing space available at the SPUI.   In this example, Reisterstown Road was widened to accommodate both the SPUI left turn lanes and the left-turn lane at the adjacent intersection.  With a Parclo B4, the design is conducive of longer left-turn storage space since the off-ramp signals need to be a far distance apart to accommodate the loop ramps (OTOH, with a SPUI the adjacent signal may only be a couple hundred feet away). . 

2.  At a SPUI, all traffic exiting the freeway does so from one off-ramp.  This increases the likelihood that traffic will back up onto the freeway.  With a Parclo B4, there are two off-ramps effectively doubling the queue space for exiting freeway traffic (reducing the likelihood that traffic will back up onto the freeway).


https://www.google.com/maps/@39.3848461,-76.7346219,589m/data=!3m1!1e3

[froggie]

Regarding your point #2, a Parclo B4 wouldn't help much in this case...it's pretty clear that the vast majority of traffic on the off-ramp is trying to head north on Reisterstown Rd.  That's a queue that would likely back up onto the Beltway even if this was a Parclo B4.

[tradephoric]

Regarding your point #2, a Parclo B4 wouldn't help much in this case...it's pretty clear that the vast majority of traffic on the off-ramp is trying to head north on Reisterstown Rd.  That's a queue that would likely back up onto the Beltway even if this was a Parclo B4.

I'm counting about 30 vehicles making a left turn off I-695 (and roughly 60 waiting to turn right).  If you reduce the queue by 33%, that's significant.

[lordsutch]

I'm counting about 30 vehicles making a left turn off I-695 (and roughly 60 waiting to turn right).  If you reduce the queue by 33%, that's significant.

Or, y'know, you could just add another lane to the off-ramp and make it a optional-lane exit.

Without investigating the interchange in detail, not allowing the right-hand off-ramp lane to free-flow into the third lane that shows up 20 feet down the road also seems to be a serious deficiency that appears to have been deliberately introduced.

[tradephoric]

Or, y'know, you could just add another lane to the off-ramp and make it a optional-lane exit.

Without investigating the interchange in detail, not allowing the right-hand off-ramp lane to free-flow into the third lane that shows up 20 feet down the road also seems to be a serious deficiency that appears to have been deliberately introduced.

Imagine a scenario where an off-ramp signal is stuck red during the PM rush.  Would a SPUI or a Parclo B4 better manage this scenario?  All traffic exiting the freeway at a Parclo B4 make a simple right-turn onto the arterial.  That's a big advantage.  There's a reason why there are many unsignalized Parclo B4 interchanges out there.  It's just a simple design that reduces the likelihood that traffic will back up onto the freeway.  Can you cite even one example of an unsignalized SPUI?

[cl94]

Or, y'know, you could just add another lane to the off-ramp and make it a optional-lane exit.

Without investigating the interchange in detail, not allowing the right-hand off-ramp lane to free-flow into the third lane that shows up 20 feet down the road also seems to be a serious deficiency that appears to have been deliberately introduced.

Imagine a scenario where an off-ramp signal is stuck red during the PM rush.  Would a SPUI or a Parclo B4 better manage this scenario?  All traffic exiting the freeway at a Parclo B4 make a simple right-turn onto the arterial.  That's a big advantage.  There's a reason why there are many unsignalized Parclo B4 interchanges out there.  It's just a simple design that reduces the likelihood that traffic will back up onto the freeway.  Can you cite even one example of an unsignalized SPUI?

Except if something is stuck on red for that long, there's a good chance it would register as a fault and send the thing into flash mode. Not ideal, I know, but many B4s are signalized with NTOR from the ramp. You'd have some of the same issues. A SPUI is almost always superior to a B4 just because the traffic entering the arterial is metered.

[tradephoric]

Except if something is stuck on red for that long, there's a good chance it would register as a fault and send the thing into flash mode. Not ideal, I know, but many B4s are signalized with NTOR from the ramp. You'd have some of the same issues. A SPUI is almost always superior to a B4 just because the traffic entering the arterial is metered.

Here's an example of a Parclo B4 with metered off ramps.  Also, all traffic is allowed to make a right turn on red. 
https://www.google.com/maps/@29.54,-95.1423654,686m/data=!3m1!1e3

[Atomica]

Sounds to me like that Parclo could be affected with some of the same issues cross traffic drivers faced in Santa Barbara when US101 was signalised through downtown prior to 1991.

[PurdueBill]

Or, y'know, you could just add another lane to the off-ramp and make it a optional-lane exit.

Without investigating the interchange in detail, not allowing the right-hand off-ramp lane to free-flow into the third lane that shows up 20 feet down the road also seems to be a serious deficiency that appears to have been deliberately introduced.

Imagine a scenario where an off-ramp signal is stuck red during the PM rush.  Would a SPUI or a Parclo B4 better manage this scenario?  All traffic exiting the freeway at a Parclo B4 make a simple right-turn onto the arterial.  That's a big advantage.  There's a reason why there are many unsignalized Parclo B4 interchanges out there.  It's just a simple design that reduces the likelihood that traffic will back up onto the freeway.  Can you cite even one example of an unsignalized SPUI?

Here's one, although it's admittedly on a dying expressway where the cross street might have more traffic than the expressway. 

[tradephoric]

Here's one, although it's admittedly on a dying expressway where the cross street might have more traffic than the expressway. 

I'm impressed Bill!  How did u stumble upon this unsignalized SPUI?  Here are some unsignalized Parclo B4 interchanges out there:

https://www.google.com/maps?ll=39.66336,-77.73976&z=17&t=h
https://www.google.com/maps/@34.72814,-86.6702,1296m/data=!3m1!1e3
https://www.google.com/maps?ll=43.17946,-71.53100&z=17&t=h
https://www.google.com/maps/@40.77906,-82.47048,1194m/data=!3m1!1e3
https://www.google.com/maps/@52.142707,-113.8139833,484m/data=!3m1!1e3
https://www.google.com/maps/@40.87241,-77.77835,1193m/data=!3m1!1e3
https://www.google.com/maps/@40.82006,-98.37846,1194m/data=!3m1!1e3
https://www.google.com/maps/@34.09093,-81.16557,653m/data=!3m1!1e3
https://www.google.com/maps/@39.93293,-76.68957,605m/data=!3m1!1e3
https://www.google.com/maps/@41.6661,-71.0932,589m/data=!3m1!1e3
https://www.google.com/maps/@35.76171,-78.02815,1280m/data=!3m1!1e3
https://www.google.com/maps/@41.1113,-100.76341,1188m/data=!3m1!1e3
https://www.google.com/maps/@46.75404,-71.42404,1081m/data=!3m1!1e3
https://www.google.com/maps/@35.0824,-85.20656,645m/data=!3m1!1e3
https://www.google.com/maps/@43.15426,-86.21083,1151m/data=!3m1!1e3
https://www.google.com/maps/@47.60011,-52.73229,1063m/data=!3m1!1e3
https://www.google.com/maps/@39.15734,-76.60406,611m/data=!3m1!1e3
https://www.google.com/maps/@37.75684,-79.40965,1247m/data=!3m1!1e3
https://www.google.com/maps/@42.9905,-82.34378,1154m/data=!3m1!1e3
https://www.google.com/maps/@42.2932,-82.92605,583m/data=!3m1!1e3
https://www.google.com/maps/@46.42746,-81.12091,1087m/data=!3m1!1e3
https://www.google.com/maps/@38.1583,-84.89337,1240m/data=!3m1!1e3
https://www.google.com/maps/@41.60732,-87.10798,1179m/data=!3m1!1e3
https://www.google.com/maps/@30.4853,-84.15839,1359m/data=!3m1!1e3
https://www.google.com/maps/@37.19931,-77.46347,1256m/data=!3m1!1e3
https://www.google.com/maps/@30.41204,-81.74739,1360m/data=!3m1!1e3
https://www.google.com/maps/@30.11086,-81.44671,1364m/data=!3m1!1e3
https://www.google.com/maps/@30.53699,-88.23993,679m/data=!3m1!1e3
https://www.google.com/maps/@39.66267,-83.94955,1214m/data=!3m1!1e3
https://www.google.com/maps/@34.39711,-92.82724,1301m/data=!3m1!1e3
https://www.google.com/maps/@39.30473,-85.96192,1220m/data=!3m1!1e3
https://www.google.com/maps/@39.83323,-84.81687,1211m/data=!3m1!1e3
https://www.google.com/maps/@61.22703,-149.73356,380m/data=!3m1!1e3
https://www.google.com/maps/@40.76559,-73.68983,597m/data=!3m1!1e3
https://www.google.com/maps/@40.83691,-96.71197,1193m/data=!3m1!1e3
https://www.google.com/maps/@37.97293,-84.8753,1243m/data=!3m1!1e3
https://www.google.com/maps/@44.03049,-92.48802,1134m/data=!3m1!1e3
https://www.google.com/maps/@37.85138,-122.0285,623m/data=!3m1!1e3

And here's an unsignalized Parclo B4 servicing a major 6-lane boulevard:
https://www.google.com/maps/@27.90329,-82.3428,1394m/data=!3m1!1e3

[tradephoric]

Here's one, although it's admittedly on a dying expressway where the cross street might have more traffic than the expressway. 

By the way, this is a perfect example of how adjacent intersections can limit the left-turn queuing space.  The adjacent intersection is only 300 feet away from the SPUI, and the left-turn lane is only 50 feet long (just enough space for 2 or 3 vehicles to squeeze in).

[jakeroot]

Here's one, although it's admittedly on a dying expressway where the cross street might have more traffic than the expressway. 

I'm impressed Bill!  How did u stumble upon this unsignalized SPUI?3

I was so close to finding this intersection, but instead I found the northern end of the Amstutz, which is also a stop-sign controlled half SPUI (though it's technically an abandoned full SPUI). I didn't bother to look any farther south. I kind of wish I had now.

FWIW, these are a couple of the oldest SPUIs in the country (I believe the first one was in Florida, but both the one in Florida and these in Illinois were built in the 70s).

[tradephoric]

One of the largest DDI's is about to be constructed in Florida.  With so much through traffic along the main corridor, is this really the best location for a DDI?  Got to love that green-to-red signal progression drivers get to enjoy.  What a thrill.

https://www.youtube.com/watch?v=24jMVZszPTY#

[NE2]

One of the largest DDI's is about to be constructed in Florida.  With so much through traffic along the main corridor

How do you know it's not turning off within a mile of the interchange?

PARCLO B4

[tradephoric]

One of the largest DDI's is about to be constructed in Florida.  With so much through traffic along the main corridor

How do you know it's not turning off within a mile of the interchange?

PARCLO B4

I agree.  A Parclo B4 would be a better choice for this interchange.  Thank you.

[cl94]

One of the largest DDI's is about to be constructed in Florida.  With so much through traffic along the main corridor

How do you know it's not turning off within a mile of the interchange?

PARCLO B4

I agree.  A Parclo B4 would be a better choice for this interchange.  Thank you.

Not enough room for it. Only quadrant that could easily fit a loop with a decent radius. The DDI could be built within the footprint of the current interchange, while a B4 would require relocation of retention ponds (not easy) and land acquisition. 

[tradephoric]

Not enough room for it. Only quadrant that could easily fit a loop with a decent radius. The DDI could be built within the footprint of the current interchange, while a B4 would require relocation of retention ponds (not easy) and land acquisition. 

Just extend the loop ramp farther out.  It would require more ROW acquisition but the advantage is the I-75 bridge would not need to be widened to accommodate the deceleration lane.  Even if a Parclo B4 costs more money, it would have operational advantages over the DDI.  Cost isn't the only factor when deciding which type of interchange design to choose (and for all we know the Parclo B4 would be cheaper).