Double left turns with permissive phasing

[swbrotha100]

I lived in Tucson for years. Still visit once in awhile. I have seen the PPLT in practice. Maybe it would be different if Tucson used the flashing yellow left signals more. AFAIK the only flashing yellow left arrows in use are on Grant Rd on either side of Oracle Rd as part of the indirect left turn setup there. I believe Tucson insists on permissive dual left signals because most of them are lagging left turn, and because of the lack of freeways/highways, most of the arterials have to pick up the slack in terms of moving traffic around the city.

[UCFKnights]

I personally think the permissive dual left turns should be a rarity instead of the norm in places like Tucson.

But why? Is there any evidence to suggest that Tucson needs to do away with their permissive dual left turns? Tucson's website is highly protective of the permissive phase:

Protected Left Turn Arrows

Protected left turn signals include a red arrow along with the normal green and amber arrow. They allow left turning drivers to proceed only on the green arrow. This turning method is very inefficient and generally not used in Tucson. Adding inefficiencies to signal timing reduces overall capacity and increases congestion. With increased congestion comes the potential for an increase in certain types of accidents.

Permitted/Protected Left Turn Arrows

This is the most common turning method used in Tucson at locations having left turn arrows. During the permitted "green ball" part of the cycle, vehicles are allowed to turn when there are adequate gaps in opposing traffic. This type of left turn phasing is designed to help minimize delay by eliminating the need for the red arrow and allowing vehicles to turn on the green ball after opposing traffic has cleared. By not having the red arrow, motorists do not have to sit and wait to turn left even when there is no opposing traffic, a situation that often occurs during periods of low traffic volumes. The signal still provides a green left turn arrow for those not able to turn during the permitted phase.

I think one thing people who have never been to Tucson should realize that most of the Tucson/Pima County area uses lagging left turns at their signalized intersections.

On leading vs lagging signals, with the advent of flashing yellow arrows, protected/permissive left turns should begin with the permissive phase. If there's only one car, they'll get through either in the middle of the cycle, or at the end. And if several more cars show up to turn left, the lagging protected left turn will be able to get the rest of them through the intersection, rather than making them wait for the next leading left turn, which would be after the cross-traffic gets their phase.

My point being, Tucson is pretty ingenious, having adopted lagging pro/per signals. Though I'd like to see them transition over to FYAs, so there isn't a red phase in the middle of the left turns.

I find that often if both directions get lagging lefts, there frequently is one car on one side and a whole group on the other, and the side with one car is forced to wait with a green arrow while all the opposing left turns happen, wasting time. I prefer when they do the direction with more left turn traffic leading, and the direction with less traffic lagging.

[tradephoric]

A lot of agencies don't like lagging lefts at permissive signals because they can lead to a perceived yellow trap.  The only way to prevent the perceived yellow trap is to tie the left-turns together (so that the left turns come on and terminate together).  While this may be safer, it is quite inefficient if you have uneven left-turn queues.  Here's an example of a perceived yellow trap at 38 seconds in the video (when a left turning driver sees the thru signal they are facing turn yellow, they mistakenly believe the opposing thru is turning yellow too).

https://www.youtube.com/watch?v=iW4xx11CLD0

[cl94]

I agree completely. Of course, the yellow trap is less of an issue if FYAs are used. NYSDOT Region 1 often staggers protected turn phases, having one direction leading and the other lagging. This eliminates the issue with uneven queues, but it can cause confusion.

[tradephoric]

The FYA has pretty much solved the "yellow trap"  problem but it doesn't prevent the perceived yellow trap.  The staggered signal phasing of NYSDOT Region 1 still leads to a perceived yellow trap (assuming the left turns are PPLTs and not protected only).   It comes down to some agencies are fine with perceived yellow traps and some never want to see it.

Here's a snippet from a webinar by the Oregon Transportation Research and Education Consortium about whether or not the FYA solves the yellow trap:

Q: What degree does the FYA solve the "yellow trap"  problem. And does Washington County lead the protected interval at FYA or lead/lag to benefit coordination?

A: The FYA completely solves the yellow trap problem, because now the left turn lane has its own display.  There is still the potential of the "perceived yellow trap"  which is when the driver in the left lane sees the adjacent through go solid yellow and thinks that the FYA is also ending.  It is called perceived, because the driver should focus on their own lane's signal control. Washington County prefers to lead the protected interval at FYA so if the protected left isn't needed then it can be skipped.  In a coordinated system we will lead or lag whichever benefits the coordination. (SS)

[jakeroot]

I don't know if the perceived yellow trap is really so much of an issue that lead/lag phasing should be discouraged. But, using more than one left turn FYA may help in making sure drivers understand which signals are for which movement. Oregon, and Washington, are notorious for using the bare minimum number of signals, with supplementary signals used only when required by sight-distance issues. Both of these states could benefit from additional supplementary signals, both for the through movements, and the turn movements.

The gold standard, as far as I'm concerned, is British Columbia, which requires three left turn heads for protected left turn movements. This kind of setup, with plenty of additional signals to bounce of off, should keep drivers focused on their signals, and not the through signals. I get the feeling that this style of setup, used with flashing yellow arrows, would prove far more successful than the typical OR/WA setup (or any other state that uses too few signals).

[cl94]

The FYA has pretty much solved the "yellow trap"  problem but it doesn't prevent the perceived yellow trap.  The staggered signal phasing of NYSDOT Region 1 still leads to a perceived yellow trap (assuming the left turns are PPLTs and not protected only).   It comes down to some agencies are fine with perceived yellow traps and some never want to see it.

Here's a snippet from a webinar by the Oregon Transportation Research and Education Consortium about whether or not the FYA solves the yellow trap:

Q: What degree does the FYA solve the "yellow trap"  problem. And does Washington County lead the protected interval at FYA or lead/lag to benefit coordination?

A: The FYA completely solves the yellow trap problem, because now the left turn lane has its own display.  There is still the potential of the "perceived yellow trap"  which is when the driver in the left lane sees the adjacent through go solid yellow and thinks that the FYA is also ending.  It is called perceived, because the driver should focus on their own lane's signal control. Washington County prefers to lead the protected interval at FYA so if the protected left isn't needed then it can be skipped.  In a coordinated system we will lead or lag whichever benefits the coordination. (SS)

R1 only uses the staggered phasing if it's fully-protected. PPLT gets leading if both directions, either lagging or leading if one direction.

[paulthemapguy]

There's also the assumption that protected-permitted left turns will see drivers poking their nose into the intersection before turning left, yes?  If we're dealing with a signal that isn't a lagging left (which is almost always the case), those lingering left-turners are going to book it out of the intersection when the signal changes to a yellow ball and a red ball.  I'm not terribly confident that, in situations with dual left turn lanes, those two drivers are going to jet out of the intersection in an orderly and safe fashion.  Just my 2 cents.

The gold standard, as far as I'm concerned, is British Columbia, which requires three left turn heads for protected left turn movements...

And yet the left turn signals still have red balls instead of red arrows  I might be the only one who gets bothered by that, though (you see it everywhere too)

[jakeroot]

The gold standard, as far as I'm concerned, is British Columbia, which requires three left turn heads for protected left turn movements...

And yet the left turn signals still have red balls instead of red arrows  I might be the only one who gets bothered by that, though (you see it everywhere too)

That's a Canada thing. I can't honestly say I've ever seen a red arrow in BC before. Can't speak for the rest of the provinces, though I don't recall seeing any in Alberta last I was there.

[mariethefoxy]

there is one I know of, its in Syosset NY on S Oyster Bay Road and the I-495 (LIE) Westbound Service Road.

[jakeroot]

I think PPLT situations should always have the permissive phase at the end. Cuts down on cycle time, as traffic conditions might not warrant a permissive phase for the vehicles waiting to turn. It is not very common in much of the northeast, but NYSDOT Region 1 uses lagging phases extensively, especially if only one direction gets a protected phase. I do not know if the region's FYA intersections are leading or lagging.

Sorry to bring up such an old quote, but I need some clarification. When you said "permissive" in the first two sentences, did you mean "protected"? It makes more sense with the word "protected", especially since you brought up Region 1's lagging phasing immediately after:

I think PPLT situations should always have the *protected* phase at the end. Cuts down on cycle time, as traffic conditions might not warrant a *protected* phase for the vehicles waiting to turn...

[Brandon]

The gold standard, as far as I'm concerned, is British Columbia, which requires three left turn heads for protected left turn movements.

Not unlike Illinois which requires two (minimum) signals per turning direction and three (minimum) for through traffic.

An example with permitted lefts (IDOT District 3): https://goo.gl/maps/5qq6x6dJKjo
An example with protected only lefts (IDOT District 1): https://goo.gl/maps/kXSRrhtd5iL2

[jakeroot]

The gold standard, as far as I'm concerned, is British Columbia, which requires three left turn heads for protected left turn movements.

Not unlike Illinois which requires two (minimum) signals per turning direction and three (minimum) for through traffic.

I've noticed that Illinois uses a lot of overhead stop-line signals (when five-section signals are involved). Can't say I'm a huge fan of this practice (I prefer post-mounted stop line signals, which I think are more common in Illinois anyways), but at least there's more than one signal, unlike out here in Washington and Oregon, where you're lucky to have more than one signal for turning movements (hence my preference for the BC standard).

From my experience, these places have the best signals:

- British Columbia (+1 for three pole-mounted protected left turns)
- California (-1 for their propensity to use protected lefts, but +1 for near-side pole mounted signals for through movements)
- Tucson, AZ (+1 for dual permissive lefts and near-side pole-mounted signals)
- Illinois (-1 when near-side overhead signals are used)
- Wisconsin (+1 for near-side pole mounted signals at nearly every junction)
- Colorado (+1 for overhead signals using backplates but not side-mounted signals -- I think this looks better and makes the mast-arms less bulky).

[roadfro]

OT reply...

- Colorado (+1 for overhead signals using backplates but not side-mounted signals -- I think this looks better and makes the mast-arms less bulky).

Terms might be a bit jumbled here... The "mast arm" is the part of the traffic signal assembly that sticks out over the roadway (typically horizontally, but can also be curved as seen in California). The "mast" is the vertical pole to which the mast arm (and side-mounted signals, if present) are attached. So a side-mounted signal head missing a backplate wouldn't affect the look of a mast arm.

[coatimundi]

It seems to have changed based on my more recent visits, but it used to be that, within the City of Tucson, a dual lane left turn could be taken on the solid green, yielding to oncoming traffic while, outside the city limits in unincorporated Pima County, while a single lane left could be taken on a solid green, a double was only on the arrow. I never really noticed it, but the local daily has a roads column, and it once published a question about it, with a county traffic engineer indicating that it was actually a rule.
Always thought that was weird.

[7/8]

I believe these aren't allowed in Ontario. The way we currently signal dual left turns doesn't allow for this to even exist, since we don't use the FYA. Once the protected turn is over, the dual left turn lanes always face a red light (see interchange below).
https://www.google.ca/maps/@43.4284371,-80.4322768,3a,30y,40.12h,95.56t/data=!3m6!1e1!3m4!1sbHdd-94cz3CK0fKRHOfXEg!2e0!7i13312!8i6656

Dual left turn lanes are often found at busy intersections from what I've noticed, so I don't think allowing this in Ontario would be worth the increased risk of side-swipes and hitting pedestrians.

[JMAN_WiS&S]

Clairemont Ave (US 12) in Eau Claire Wisconsin has dual left turns across 3 lanes of 45mph traffic and two intersections like this are getting protected/permissive FYA. not sure how it will go..

[jakeroot]

OT reply...

- Colorado (+1 for overhead signals using backplates but not side-mounted signals -- I think this looks better and makes the mast-arms less bulky).

Terms might be a bit jumbled here... The "mast arm" is the part of the traffic signal assembly that sticks out over the roadway (typically horizontally, but can also be curved as seen in California). The "mast" is the vertical pole to which the mast arm (and side-mounted signals, if present) are attached. So a side-mounted signal head missing a backplate wouldn't affect the look of a mast arm.

Whoops. Yes I meant "mast". Thanks for filling me in.

[jakeroot]

Just gonna dust off this thread. Looking for some input from those familiar with Maryland.

I found a left turn in Silver Spring (Route 650 onto Columbia Pike). It's a dual left turn with a flashing red arrow, with turns permitted after a full stop.

I've read before that flashing red arrows are pretty ubiquitous in Maryland (versus flashing yellow arrows). I can tell from Street View that many single lefts have FRAs, but not many dual lefts.

Anyone know how often Maryland installs dual left flashing red arrows?

Here is the Gmaps link to the intersection below: https://goo.gl/Lr4gDD -- according to Historic Aerials, the second left turn lane was added in 2005.

[mrsman]

Just gonna dust off this thread. Looking for some input from those familiar with Maryland.

I found a left turn in Silver Spring (Route 650 onto Columbia Pike). It's a dual left turn with a flashing red arrow, with turns permitted after a full stop.

I've read before that flashing red arrows are pretty ubiquitous in Maryland (versus flashing yellow arrows). I can tell from Street View that many single lefts have FRAs, but not many dual lefts.

Anyone know how often Maryland installs dual left flashing red arrows?

Here is the Gmaps link to the intersection below: https://goo.gl/Lr4gDD -- according to Historic Aerials, the second left turn lane was added in 2005.

While not an expert on MD per se, I am very familiar with this intersection as I drive by frequently.

I can say that this an exception as I cannot think of other permissive double lefts in MD anywhere.  One factor how this may be safer than others is that this is a
T intersection.  There is no corresponding left turn in the opposite direction, and you don't have to worry about cross traffic or right turns or anything.  So if there are other dual left turn FRAs, they would only be in similar circumstances (on-ramp to freeway) and not a standard intersection.

FWIW, the visibility here is excellent. It is very easy to see SB traffic when making the left turn from NB NH ave to US 29 (from either lane).  I believe before 2005, there was no traffic signal at all and for most hours of the day, except rush hour a singal was not needed.  It seemed that there was enough of a backup during rush hours that they felt that a second lane was necessary and still provided an option for a protected signal.  I give DOT a lot of credit for making this protected/permissive.

The normal operation of the signal is SB green and NB FRA.  When there are enough who want to make the left, the SB signal goes from yellow to red.  After a brief period NB left becomes a green arrow, then a yellow arrow, and then a solid red arrow for a few seconds while SB traffic is released with a green.  And then FRA again.

[jakeroot]

Just gonna dust off this thread. Looking for some input from those familiar with Maryland.

I found a left turn in Silver Spring (Route 650 onto Columbia Pike). It's a dual left turn with a flashing red arrow, with turns permitted after a full stop.

I've read before that flashing red arrows are pretty ubiquitous in Maryland (versus flashing yellow arrows). I can tell from Street View that many single lefts have FRAs, but not many dual lefts.

Anyone know how often Maryland installs dual left flashing red arrows?

Here is the Gmaps link to the intersection below: https://goo.gl/Lr4gDD -- according to Historic Aerials, the second left turn lane was added in 2005.

While not an expert on MD per se, I am very familiar with this intersection as I drive by frequently.

I can say that this an exception as I cannot think of other permissive double lefts in MD anywhere.  One factor how this may be safer than others is that this is a
T intersection.  There is no corresponding left turn in the opposite direction, and you don't have to worry about cross traffic or right turns or anything.  So if there are other dual left turn FRAs, they would only be in similar circumstances (on-ramp to freeway) and not a standard intersection.

FWIW, the visibility here is excellent. It is very easy to see SB traffic when making the left turn from NB NH ave to US 29 (from either lane).  I believe before 2005, there was no traffic signal at all and for most hours of the day, except rush hour a singal was not needed.  It seemed that there was enough of a backup during rush hours that they felt that a second lane was necessary and still provided an option for a protected signal.  I give DOT a lot of credit for making this protected/permissive.

The normal operation of the signal is SB green and NB FRA.  When there are enough who want to make the left, the SB signal goes from yellow to red.  After a brief period NB left becomes a green arrow, then a yellow arrow, and then a solid red arrow for a few seconds while SB traffic is released with a green.  And then FRA again.

I'm really, really glad to hear these work well. Anybody who knows me, knows I really like permissive double lefts (mostly because I don't like protected lefts, period). Maryland's approach seems like a really good middle ground between flashing yellow, and protected-only. Particularly at Parclo B4s (such as in the Silver Spring example), where the only signalised movement is a left turn, they seem like a great candidate for FRAs (though only if the left is a double left -- I still prefer FYAs at single left turns.

As for the phasing, I'd have thought that going from FRA to green arrow would be acceptable (FRA to solid yellow seems a bit odd). But if the phasing described above is what Maryland has more success with, more power to them.

[mrsman]

Just gonna dust off this thread. Looking for some input from those familiar with Maryland.

I found a left turn in Silver Spring (Route 650 onto Columbia Pike). It's a dual left turn with a flashing red arrow, with turns permitted after a full stop.

I've read before that flashing red arrows are pretty ubiquitous in Maryland (versus flashing yellow arrows). I can tell from Street View that many single lefts have FRAs, but not many dual lefts.

Anyone know how often Maryland installs dual left flashing red arrows?

Here is the Gmaps link to the intersection below: https://goo.gl/Lr4gDD -- according to Historic Aerials, the second left turn lane was added in 2005.

While not an expert on MD per se, I am very familiar with this intersection as I drive by frequently.

I can say that this an exception as I cannot think of other permissive double lefts in MD anywhere.  One factor how this may be safer than others is that this is a
T intersection.  There is no corresponding left turn in the opposite direction, and you don't have to worry about cross traffic or right turns or anything.  So if there are other dual left turn FRAs, they would only be in similar circumstances (on-ramp to freeway) and not a standard intersection.

FWIW, the visibility here is excellent. It is very easy to see SB traffic when making the left turn from NB NH ave to US 29 (from either lane).  I believe before 2005, there was no traffic signal at all and for most hours of the day, except rush hour a singal was not needed.  It seemed that there was enough of a backup during rush hours that they felt that a second lane was necessary and still provided an option for a protected signal.  I give DOT a lot of credit for making this protected/permissive.

The normal operation of the signal is SB green and NB FRA.  When there are enough who want to make the left, the SB signal goes from yellow to red.  After a brief period NB left becomes a green arrow, then a yellow arrow, and then a solid red arrow for a few seconds while SB traffic is released with a green.  And then FRA again.

I'm really, really glad to hear these work well. Anybody who knows me, knows I really like permissive double lefts (mostly because I don't like protected lefts, period). Maryland's approach seems like a really good middle ground between flashing yellow, and protected-only. Particularly at Parclo B4s (such as in the Silver Spring example), where the only signalised movement is a left turn, they seem like a great candidate for FRAs (though only if the left is a double left -- I still prefer FYAs at single left turns.

As for the phasing, I'd have thought that going from FRA to green arrow would be acceptable (FRA to solid yellow seems a bit odd). But if the phasing described above is what Maryland has more success with, more power to them.

I may not have been clear in my last post about the signaling.  Hopefully this is better:

NB:                         SB:

FRA                         Green
FRA                         Yellow
FRA                          Red (one second)
green arrow               Red
yellow arrow              red
solid red arrow           red (one second)
solid red arrow           green (15 seconds)
FRA                          green

There is a brief period, about 15 seconds, when SB traffic is first released on the green that NB traffic may not turn left (solid red arrow).

There is no interval from FRA to yellow arrow.  There is no indication from the NB side that SB gets a yellow ball.  You sit at the intersection waiting to make a left and you may notice that some of the cars that are approaching SB are slowing or stopping.  By the time you realize that the opposing side has a red, the green arrow comes on and you can go.

Of course NB thru traffic faces no signal whatsoever and is functionally equivalent to a continuous green.

[jakeroot]

There is no interval from FRA to yellow arrow.  There is no indication from the NB side that SB gets a yellow ball.  You sit at the intersection waiting to make a left and you may notice that some of the cars that are approaching SB are slowing or stopping.  By the time you realize that the opposing side has a red, the green arrow comes on and you can go.

I did misunderstand what you wrote. That you for the clarification. I got the SB and NB phasings mixed up.

[jakeroot]

Here's a dual permissive left in Brooklyn, NY (a state with very few dual permissive lefts). The left turn signal is a traditional 5-section tower (common in NYC? I'm not sure):

Flatbush @ Tillary, Brooklyn

[cl94]

Here's a dual permissive left in Brooklyn, NY (a state with very few dual permissive lefts). The left turn signal is a traditional 5-section tower (common in NYC? I'm not sure):

Flatbush @ Tillary, Brooklyn

The 5-section is the standard permissive signal in New York City. Another type is a 2-section 12" arrow face alongside a 3-section 8" face. I don't think there's a single doghouse inside the City.

Having family nearby until relatively recently, this was the only dual-permissive left turn installation I knew of anywhere until I was almost 9. It's still one of the less than 5 I have seen in person. I'm convinced that this was only done because opposing turns are prohibited and sightlines are wide-open. I do NOT know of another dual permissive left in New York that has opposing left turns (excluding the east end of Queens Blvd, which has open sightlines due to the way the WB ROW shifts).