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National Boards => General Highway Talk => Topic started by: tradephoric on February 20, 2018, 01:05:51 AM

Title: Contraflow left-turn intersection
Post by: tradephoric on February 20, 2018, 01:05:51 AM
Who needs dedicated left turn lanes when you can just use opposing thru lanes of traffic? 

Quote
This is the footage of an unconventional left-turn treatment called "Contraflow Left Turn", in which the left turn flow is allowed to use the opposite lanes with the help of a pre-signal, this gives additional two lanes for the left turning vehicles without any massive infrastructure renewal. Although this design increases the capacity of the left-turning phase, safety, driver acceptance, and other issues should be addressed before extensive application. It can also be said that this is an intermediate solution between the classic design and "Displaced Left-Turn Intersection".

Title: Re: Contraflow left-turn intersection
Post by: jakeroot on February 20, 2018, 02:33:43 AM
That seems like a remarkably stupid idea. But speeds seem low enough that the chance of a crash is likely pretty slim. I'd prefer a proper displaced left turn.
Title: Re: Contraflow left-turn intersection
Post by: kj3400 on February 20, 2018, 05:22:40 AM
I feel like it would work better with a protected green. You can clearly see both through directions yielding to turning traffic. While it would increase throughput for turning traffic, if you need to do that, maybe it's time to consider different options for your intersection.
Title: Re: Contraflow left-turn intersection
Post by: Brandon on February 20, 2018, 07:07:48 AM
We already have that in Chicago.  Granted, it's not exactly legal, but a fair number of drivers already use the opposing lanes to get to their left turn lane, or use it to bypass heavy traffic and just turn left to a side street/driveway.
Title: Re: Contraflow left-turn intersection
Post by: kalvado on February 20, 2018, 08:21:23 AM
If there is no room for an additional lane - what can you do? Using contraflow lane for protected-only phase (as far as I understand, it is protected-only) should be OK if there is now contraflow backup bad enough to extend for entire block. I don't see a systematic  accident scenario here.
And this all assumes that cross street has 2 independent lanes. We do have some (quite stupidly designed) intersections where 2 turn lanes funnel into a single street lane. More hindrance than help.
And traffic on a cross-street may choke due to downstream intersections throughput. After all, flow is only as good as the lowest throughput point..
Title: Re: Contraflow left-turn intersection
Post by: johndoe on February 20, 2018, 06:26:55 PM
Thanks for sharing, here are some other discussions:
https://www.fhwa.dot.gov/publications/research/operations/18016/18016.pdf
https://www.fhwa.dot.gov/publications/research/operations/16064/006.cfm
Title: Re: Contraflow left-turn intersection
Post by: Super Mateo on February 20, 2018, 07:40:22 PM
We already have that in Chicago.  Granted, it's not exactly legal, but a fair number of drivers already use the opposing lanes to get to their left turn lane, or use it to bypass heavy traffic and just turn left to a side street/driveway.

Opposing lanes, medians, curbs, and shoulders are all fair game in Chicago.  And there are cases where drivers will turn from through (straight only) lanes just to go around the traffic.
Title: Re: Contraflow left-turn intersection
Post by: paulthemapguy on February 22, 2018, 08:49:35 PM
This is an interesting idea for roads with quite a few lanes, like this 6-lane example.  If there are 3 thru lanes in each approach to the intersection, and only one left turn lane, then you only need to use one of the three lanes leaving the intersection during left-turn phases.  We can make use of the two remaining lanes in this case, to temporarily house the left-turn traffic.  It's a more efficient use of space, if we can get drivers to understand what the heck is going on, and if the signals are phased in the right sequence.

My problem is-- doesn't this limit the movements that can be allowed during a signal phase?  A 4-way intersection needs 4 phases to permit all possible movements.  If the contraflow intersection necessitates any phases limiting the procession of traffic to one side at a time, whether left turn or straight, that increases the number of phases, and therefore, wait times.  More wait times = less efficiency.  And you have to clear one direction's left turn lanes before the opposite approach's straight movement.  Idk
Title: Re: Contraflow left-turn intersection
Post by: formulanone on February 22, 2018, 10:41:08 PM
Coconut Creek, Florida sort of has one of these, but they're dedicated turn lanes.
https://www.google.com/maps/@26.3013384,-80.1863529,430m/data=!3m1!1e3

Here's a northbound Lyons Road view:
(https://farm5.staticflickr.com/4699/40431421801_9cbf9ca591_b.jpg) (https://flic.kr/p/24AMB6k)

Northbound, underneath FL 869:
(https://farm5.staticflickr.com/4604/39720812394_051309c8bc_b.jpg) (https://flic.kr/p/23vZxF1)

This was coming from southbound on Lyons Road, in the contraflow lane:
(https://farm5.staticflickr.com/4765/39720810154_7c6f40d179_b.jpg) (https://flic.kr/p/23vZx1o)

Waiting my turn to head north ("east") on FL 869; that blue Mazda 6 is waiting in its contraflow left lane, while traffic leaves FL 869 to go north on Lyons Road:
(https://farm5.staticflickr.com/4609/26560342058_d02f16e73d_b.jpg) (https://flic.kr/p/Gt3Gm3)

I lived around this area for almost 20 years, and there wasn't much need for the added complication. Perhaps it was just experimental, because when it was built around 1985-86, there wasn't much traffic around the area, nor daily driving routines to interfere with. I can see where it allows for future growth, or added traffic to turn left. But there's the chance of additional gridlock.
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on February 23, 2018, 01:11:03 AM
Waiting my turn to head north ("east") on FL 869; that blue Mazda 6 is waiting in its contraflow left lane, while traffic leaves FL 869 to go north on Lyons Road:
(https://farm5.staticflickr.com/4609/26560342058_d02f16e73d_b.jpg) (https://flic.kr/p/Gt3Gm3)

What's up with that Cadillac SRX and that 3 series waiting mid-intersection? I don't see any stop line there. Shouldn't they have completed their turn (coming off the off-ramp)? I'm not even sure what signal they're looking at.
Title: Re: Contraflow left-turn intersection
Post by: freebrickproductions on February 23, 2018, 01:48:33 AM
Waiting my turn to head north ("east") on FL 869; that blue Mazda 6 is waiting in its contraflow left lane, while traffic leaves FL 869 to go north on Lyons Road:
(https://farm5.staticflickr.com/4609/26560342058_d02f16e73d_b.jpg) (https://flic.kr/p/Gt3Gm3)

What's up with that Cadillac SRX and that 3 series waiting mid-intersection? I don't see any stop line there. Shouldn't they have completed their turn (coming off the off-ramp)? I'm not even sure what signal they're looking at.
I think they were driving at the time. If you look closely at the silver car on the left, you can see it's finishing up a turn, along with that red Chevy. A bit of an interesting optical illusion though. :nod:
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on February 23, 2018, 01:59:03 AM
Waiting my turn to head north ("east") on FL 869; that blue Mazda 6 is waiting in its contraflow left lane, while traffic leaves FL 869 to go north on Lyons Road:

https://farm5.staticflickr.com/4609/26560342058_d02f16e73d_b.jpg

What's up with that Cadillac SRX and that 3 series waiting mid-intersection? I don't see any stop line there. Shouldn't they have completed their turn (coming off the off-ramp)? I'm not even sure what signal they're looking at.

I think they were driving at the time. If you look closely at the silver car on the left, you can see it's finishing up a turn, along with that red Chevy. A bit of an interesting optical illusion though. :nod:

Oh shit, you're right! I guess that explains why the FYA was showing a red arrow. Also why the Mazda6 is stopped.
Title: Re: Contraflow left-turn intersection
Post by: Hurricane Rex on February 23, 2018, 02:22:53 AM
I feel like one could argue a center lane as this out of confusion.

Because both people would use. I'm not saying anyone on this forum would argue that though.
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on February 23, 2018, 02:31:46 AM
I feel like one coupd argue a center lane as this out of confusion.

Sorry...what?
Title: Re: Contraflow left-turn intersection
Post by: tradephoric on February 23, 2018, 09:50:24 AM
Contraflow left turns seem to make a lot of sense at diamond interchanges as it prevents interlocking left-turn gridlock.  Also, it creates one less lane of traffic to cross for left turn drivers entering the on-ramp, potentially allowing a permissive FYA as opposed to a protected only arrow.  I'm surprised not more of these designs exist.  Here is one being built/planned in Augusta, Georgia very similar to the interchange formulanone posted:

The City of Augusta is planning to reconstruct the Wrightsboro/Interstate 520 interchange as a contraflow left interchange. Work could begin this fall.
(https://farm2.staticflickr.com/1521/24210703834_1cc4b21c03_b.jpg) (https://flic.kr/p/CTqbLb)Wrightsboro Rd/I-520 Interchange Improvement Project-Augusta (https://flic.kr/p/CTqbLb) by brandon walker (https://www.flickr.com/photos/125122378@N05/), on Flickr
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on February 23, 2018, 03:59:35 PM
Contraflow left turns seem to make a lot of sense at diamond interchanges as it prevents interlocking left-turn gridlock.  Also, it creates one less lane of traffic to cross for left turn drivers entering the on-ramp, potentially allowing a permissive FYA as opposed to a protected only arrow.  I'm surprised not more of these designs exist.

I know when I build my cities in 'Cities: Skylines', I have a hard time building this style of diamond interchange because it requires the crossing road to be rather far apart (to allow the movements to cross beforehand). The design is far superior to a standard diamond, but there's a lot of empty ROW. You can still use permissive phasing with more than one left turn lane, and across 3+ lanes of traffic. Just gotta design it properly. And use those hatches in the intersection to prevent blocking.

EDIT: In my opinion, the worst part of a diamond interchange is not the left turns onto the on-ramps, but rather the off-ramps more-or-less overlapping with each other. If the distance between the two off-ramps is rather short, it can make it rather difficult to time the lights. In one of my cities, the off-ramps are quite close, necessitating the two movements to occur one after the other. Lightly-used off-ramps (with short phases) can improve the situation, as can moving the ramp termini farther apart (though this isn't always an option).
Title: Re: Contraflow left-turn intersection
Post by: Joe The Dragon on February 23, 2018, 05:15:45 PM
Contraflow left turns seem to make a lot of sense at diamond interchanges as it prevents interlocking left-turn gridlock.  Also, it creates one less lane of traffic to cross for left turn drivers entering the on-ramp, potentially allowing a permissive FYA as opposed to a protected only arrow.  I'm surprised not more of these designs exist.

I know when I build my cities in 'Cities: Skylines', I have a hard time building this style of diamond interchange because it requires the crossing road to be rather far apart (to allow the movements to cross beforehand). The design is far superior to a standard diamond, but there's a lot of empty ROW. You can still use permissive phasing with more than one left turn lane, and across 3+ lanes of traffic. Just gotta design it properly. And use those hatches in the intersection to prevent blocking.

EDIT: In my opinion, the worst part of a diamond interchange is not the left turns onto the on-ramps, but rather the off-ramps more-or-less overlapping with each other. If the distance between the two off-ramps is rather short, it can make it rather difficult to time the lights. In one of my cities, the off-ramps are quite close, necessitating the two movements to occur one after the other. Lightly-used off-ramps (with short phases) can improve the situation, as can moving the ramp termini farther apart (though this isn't always an option).

You may be able to get way with using the lane-control mod I really need to get back into Cities: Skylines
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on February 23, 2018, 05:30:43 PM
Contraflow left turns seem to make a lot of sense at diamond interchanges as it prevents interlocking left-turn gridlock.  Also, it creates one less lane of traffic to cross for left turn drivers entering the on-ramp, potentially allowing a permissive FYA as opposed to a protected only arrow.  I'm surprised not more of these designs exist.

I know when I build my cities in 'Cities: Skylines', I have a hard time building this style of diamond interchange because it requires the crossing road to be rather far apart (to allow the movements to cross beforehand). The design is far superior to a standard diamond, but there's a lot of empty ROW. You can still use permissive phasing with more than one left turn lane, and across 3+ lanes of traffic. Just gotta design it properly. And use those hatches in the intersection to prevent blocking.

EDIT: In my opinion, the worst part of a diamond interchange is not the left turns onto the on-ramps, but rather the off-ramps more-or-less overlapping with each other. If the distance between the two off-ramps is rather short, it can make it rather difficult to time the lights. In one of my cities, the off-ramps are quite close, necessitating the two movements to occur one after the other. Lightly-used off-ramps (with short phases) can improve the situation, as can moving the ramp termini farther apart (though this isn't always an option).

You may be able to get way with using the lane-control mod I really need to get back into Cities: Skylines

Oh, trust me, I use plenty of mods. I have 144 installed, to be exact (in addition to several thousand assets). But that's not related to the problem I have with the interchange in C:S. The problem I have is that when I have to use a diamond interchange, it's usually because I don't have a  lot of ROW. So I generally go with the most compact design possible.

To be clear, I quite like the contraflow diamond interchange. My only reservation is how wide the service road needs to be throughout the interchange. Other than that, it's a great setup.
Title: Re: Contraflow left-turn intersection
Post by: johndoe on May 29, 2018, 09:54:25 PM
NCDOT has considered the concept at a few interchanges.

Here is one combined with an RCUT / J-turn: https://www.ncdot.gov/projects/i26Widening/download/i26-synchronized-interchange.pdf  (I'm not sure that I've heard of "synchronized interchange" title before...a Google search led me to this: https://digitalcommons.wayne.edu/cgi/viewcontent.cgi?article=2844&context=oa_dissertations)

Here is one combined with a Parclo B-4:  http://www.blueridgenow.com/news/20180527/beyond-balfour-parkway-is-four-seasons-fix-out-there
Title: Re: Contraflow left-turn intersection
Post by: tradephoric on May 30, 2018, 08:43:25 AM
Here is one combined with a Parclo B-4:  http://www.blueridgenow.com/news/20180527/beyond-balfour-parkway-is-four-seasons-fix-out-there


My dream interchange!  I have heard the design referred to as a "folded interchange".  Back in the day I made a Synchro model of the "folded interchange" that conceptually mimics the proposed U.S. 64/I-26 interchange.


EDIT:  I realize i started a topic about the "folded interchange" on this thread.  https://www.aaroads.com/forum/index.php?topic=8144.0  The NC design is unique in that it doesn't appear to require a huge median to fit the contrawflow left-turn movements.
Title: Re: Contraflow left-turn intersection
Post by: tradephoric on May 31, 2018, 08:27:15 AM
I'm not sure that I've heard of "synchronized interchange" title before...a Google search led me to this: https://digitalcommons.wayne.edu/cgi/viewcontent.cgi?article=2844&context=oa_dissertations

Great find there John.  Among the interchanges analyzed in the paper was the DDI, which has a much worse operational performance for both vehicles and pedestrians when compared to the Milwaukee B interchange (the only real difference between the Milwaukee B and the “folded interchange” model above is how the off-ramp loop is routed to the arterial).  You know DDI’s are being over-applied when full parclos are being converted to DDIs, even though the DDI has a much worse operational performance than the “folded interchange” (especially considering the “folded interchange” could easily fit into the existing full-parclo ROW).  But DOT’s seems to have an irrational love affair with DDI’s right now.  Even when ROWs are tight, the synchronized interchange can be a better operational alternative to the DDI when through traffic is the predominant movement.
Title: Re: Contraflow left-turn intersection
Post by: tradephoric on May 31, 2018, 09:46:31 AM
With contraflow left turns at interchanges there is potential to provide permissive left turns onto the freeway as opposed to protected only.  Some interchange on-ramp may even be able to function unsignalized when all of that opposing left turn traffic is no longer conflicting with the left-turn on-ramp traffic.  Personally, I really like interchanges that don’t require traffic signals.  At the very least have permissive left turns onto the freeway like the FYA in the video below (this is an example of a Parclo B4 interchange near Lansing… contraflow left turns would provide even more gaps in traffic as only traffic continuing through the interchange on the arterial would be conflicting with the left-turning on-ramp traffic).
 
Title: Re: Contraflow left-turn intersection
Post by: kalvado on May 31, 2018, 10:33:41 AM
  Personally, I really like interchanges that don’t require traffic signals. 
Like roundabouts?..  :hmmm:
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on May 31, 2018, 12:55:29 PM
  Personally, I really like interchanges that don’t require traffic signals. 
Like roundabouts?..  :hmmm:

Perhaps. Though, more like interchanges with minimal crossovers (such as the B4). Fewer crossovers with traditional designs means fewer signals and/or fewer phases.

FWIW, much like I stated above, traditional interchanges with left turns can (and should) use permissive phasing. Diamond interchange left turns technically deal with the same amount of traffic as the traditional B4 left turn (traffic going left onto the freeway or straight along the arterial) (assumes slip lanes for right turns).

The 15th & Dravus diamond interchange in Seattle is fully split phased, and it wreaks havoc on the area. They could seriously improve flow by disabling the "one direction of the arterial at a time" phasing ("necessary" due to the lack of left turn lanes) by allowing both directions of the arterial to proceed at the same time, and just yield at the on-ramps.
Title: Re: Contraflow left-turn intersection
Post by: tradephoric on May 31, 2018, 02:34:52 PM
The interchange names gets longer and longer.  I must agree with the website's description of this interchange being a "very efficient design" though.  Amazing really that it hasn't yet been built yet.   

Quote
4-QUADRANT 6-RAMP CONTRAFLOW LEFT PARCLO LOOPS-OFF INTERCHANGE

This is a 4-quadrant 6-ramp parclo with contraflow left turns on the crossroad.

Notice that none of the left turns interferes (or even intersects) with any of the other left turns. This is a very efficient design.

Because there is no existing example of this interchange, this is an artist's version. One is proposed in Hendersonville NC.

(https://midimagic.sgc-hosting.com/intcflpc.jpg)

https://midimagic.sgc-hosting.com/ichange.htm
Title: Re: Contraflow left-turn intersection
Post by: tradephoric on May 31, 2018, 03:02:10 PM
A presentation out of North Carolina that discusses the design of NC-54 with the focus on maintaining good signal progression.  This presentation was tailor made for me.  It even has slides explaining "Why Parclo B is Better".

(https://i.imgur.com/elkvoAd.png)

http://letsgetmoving.org/cms/wp-content/uploads/2016/04/NC-54-designing-for-progression-from-Hummer-100516.pdf

Title: Re: Contraflow left-turn intersection
Post by: jakeroot on May 31, 2018, 05:09:42 PM
This presentation was tailor made for me.

That's an understatement. If I didn't know better, I'd say you created the presentation!

Only thing I disagree with is the "all crossings signal-controlled" bit. Typical B4's, with merges or yields for the off-ramp, require drivers to give way to pedestrians. The signalised variation (without free-flow ramps) is less common, and usually more expensive to build since cloverleafs are more easily modified into the traditional free-flow design (and are often the reason the B4 exists -- prior cloverleafs).

Here in Tacoma, our B4 (former cloverleaf) operates without pedestrian crossings. A separate bridge was constructed for pedestrian crossings. But, if crosswalks were added today, this is how it would work out for pedestrians:

(https://i.imgur.com/EXVxrp6.png)

FWIW, this is by far the best interchange in the city. I have never been stuck in traffic on it -- ever. Can't say that for any other interchange.
Title: Re: Contraflow left-turn intersection
Post by: NE2 on May 31, 2018, 05:31:37 PM
That's a SPUI with two loops. There's one with one loop at the south end of the Liberty Tunnel in Pittsburgh.
Title: Re: Contraflow left-turn intersection
Post by: johndoe on July 28, 2018, 05:01:25 PM
In this PDF NCDOT talks about a "double contraflow intersection" where both grade separated routes get the unique left-turn treatment:
https://www.ncdot.gov/news/public-meetings/Documents/U-5301_Handout_Consolidated_Final.pdf
see sheet 4/14

(http://i63.tinypic.com/2s9c12v.jpg)
And notice in southeast quadrant this alternative seems to require the demolition of Peak Auto  :bigass:
Title: Re: Contraflow left-turn intersection
Post by: TheHighwayMan394 on July 28, 2018, 05:08:30 PM
Wendy's would also be inbetween an off-ramp and a freeway. Convenient for roadgeeks, but probably frowned on by FHWA.
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on July 28, 2018, 06:55:50 PM
Jesus, that's complex! Not too keen on those awkward merges from the overpass onto US-64, though. Does look to be pretty easy to configure for pedestrian crossings, however.
Title: Re: Contraflow left-turn intersection
Post by: johndoe on July 29, 2018, 10:09:27 AM
Jesus, that's complex! Not too keen on those awkward merges from the overpass onto US-64, though. Does look to be pretty easy to configure for pedestrian crossings, however.
Yeah I wonder if they're thinking of them as "ramps",  notice they have signals.  I wish their image would have ped  crossings in another color.

Also this isn't an interstate highwayman, so I'm not sure how much FHWA has to approve.
Title: Re: Contraflow left-turn intersection
Post by: Joe The Dragon on July 30, 2018, 03:33:51 PM
In this PDF NCDOT talks about a "double contraflow intersection" where both grade separated routes get the unique left-turn treatment:
https://www.ncdot.gov/news/public-meetings/Documents/U-5301_Handout_Consolidated_Final.pdf
see sheet 4/14

(http://i63.tinypic.com/2s9c12v.jpg)
And notice in southeast quadrant this alternative seems to require the demolition of Peak Auto  :bigass:
what a way to be messup a nice overpass with the left turns.
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on July 30, 2018, 05:07:43 PM
what a way to be messup a nice overpass with the left turns.

Not sure exactly what you mean, but no matter how they're configured, overpasses can seriously improve the flow through an area.
Title: Re: Contraflow left-turn intersection
Post by: Joe The Dragon on July 30, 2018, 05:24:12 PM
what a way to be messup a nice overpass with the left turns.

Not sure exactly what you mean, but no matter how they're configured, overpasses can seriously improve the flow through an area.
yes but when other plans have them with ramps why build it like the kills grade separated flow on the main line.
Title: Re: Contraflow left-turn intersection
Post by: jakeroot on July 30, 2018, 10:56:50 PM
what a way to be messup a nice overpass with the left turns.

Not sure exactly what you mean, but no matter how they're configured, overpasses can seriously improve the flow through an area.

yes but when other plans have them with ramps why build it like the kills grade separated flow on the main line.

Service interchanges require all crossing maneuvers to be on one road, which helps traffic on the mainline, but really screws with the arterial (especially tight interchanges with poor storage capacity). The interchange above really doesn't even need signals, so it's actually pretty clever. I also don't get the feeling that US 64 is supposed to be a full-blown freeway through this area, so evening the flow between the two roads is actually pretty smart. Of course, if NCDOT did decide to make 64 free flow in the future, it is easily converted into a sort of Parclo A4 interchange (make the NE and SW roads two-way, and remove the left turns from Lake Pine Dr), a pretty high capacity interchange (so long as there's enough distance between the off-ramps).
Title: Re: Contraflow left-turn intersection
Post by: tradephoric on March 18, 2019, 02:25:55 PM
This came up in the Michigan thread.  MDOT is proposing building a DDI at the I-75/Big Beaver interchange as part of the I-75 modernization project.  I personally feel like this interchange would have been a perfect candidate for a contraflow Parclo as the wide medians along Big Beaver would allow space for the contraflow lefts.  Below is the DDI being proposed along with a model of the Contraflow Parclo:

(https://s3-prod.crainsdetroit.com/s3fs-public/styles/width_792/public/Big%20Beaver%20DDI-main_i.jpg)