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[tradephoric]

Who needs dedicated left turn lanes when you can just use opposing thru lanes of traffic? 

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".

[jakeroot]

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.

[kj3400]

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.

[Brandon]

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.

[kalvado]

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..

[johndoe]

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

[Super Mateo]

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.

[paulthemapguy]

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

[formulanone]

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:


Northbound, underneath FL 869:


This was coming from southbound on Lyons Road, in the contraflow lane:


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:


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.

[jakeroot]

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:

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.

[freebrickproductions]

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:

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.

[jakeroot]

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.

Oh shit, you're right! I guess that explains why the FYA was showing a red arrow. Also why the Mazda6 is stopped.

[Hurricane Rex]

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.

[jakeroot]

I feel like one coupd argue a center lane as this out of confusion.

Sorry...what?

[tradephoric]

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.
Wrightsboro Rd/I-520 Interchange Improvement Project-Augusta by brandon walker, on Flickr

[jakeroot]

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).

[Joe The Dragon]

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

[jakeroot]

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.

[johndoe]

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

[tradephoric]

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.

[tradephoric]

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.

[tradephoric]

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).
 

[kalvado]

  Personally, I really like interchanges that don’t require traffic signals. 

Like roundabouts?.. 

[jakeroot]

  Personally, I really like interchanges that don’t require traffic signals. 

Like roundabouts?.. 

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.

[tradephoric]

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.   

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/ichange.htm