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Traffic Light Phases

Started by corco, December 12, 2010, 11:34:24 PM

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corco

In pretty much every city I have ever lived, traffic lights seem to flow in the following order at a standard intersection of two roughly equally important arterials where left turns are allowed with a green ball but there is an arrow

Arterial #1 Left Turns
Arterial #1 All traffic
Arterial #2 Left Turns
Arterial #2 All traffic

I just moved down to Tucson, however, and noticed that things are a bit different- the general flow is

Arterial #1 All traffic
Arterial #1 Left Turns
Arterial #2 All traffic
Arterial #2 Left Turns

The main advantage to this that I noticed is that if you're sitting in the middle of an intersection waiting for a left turn with a green ball and the light turns yellow, the next phase is going to be a protected left anyway so you're not sort of running the red light by hanging out in the intersection, and then once the protected phase is done traffic from the other direction gets to move.

I have never seen this before- do any other cities employ that sort of phasing?


mapman

What you're referring to is called "Permitted-Protected" (or "Permissive-Protected") signal phasing.  In California, it's relatively uncommon, but I'm not sure about the rest of the country.  Where I have seen it, it was implemented for exactly the benefit you noticed; if a number of vehicles have trouble making a left turn with just the green ball, the signal turns to the green arrow to allow the lefts to go exclusively.  There used to be a signal with that operation in downtown San Jose (on Santa Clara Street at 3rd Street), but I'm not sure if it still operates that way.

roadfro

#2
Not quite... While Corco was indicating the use of protected-permitted phasing in that descriptive paragraph, the particular question is more about the sequence of protected left turns. What Corco described in the first scenario is known as "lead-lead" turn phasing (both directions of a street get the protected left at the beginning of the cycle for that arterial), and the second scenario is "lag-lag" (both directions on a street get a protected left at the end of the cycle on that arterial).

Lead-lead is by far the most common arrangement of protected left turn phases--as such, many drivers expect the left turns to come up first for a cycle whenever a protected left turn signal is encountered.. Lead-lead is frequently seen in locations where the arterial is not part of a coordinated signal system. Lag-lag protected left turn phasing is not terribly uncommon, but many municipalities tend to not use this for whatever reason--possibly because it somewhat violates driver expectancy.

What is more common in coordinated signal systems is the use of "lead-lag" (or "lag-lead") protected left turns. This is an arrangement where one direction gets the protected left turn at the beginning and the opposite direction receives a protected left at the end of the arterial phase. Lead-lag, coupled with appropriate offsets for upstream/downstream signals, provides a greater window of time to progress platoons on an arterial. If you were to use some traffic signal timing/analysis programs (i.e. Synchro) with an arterial or maybe even a grid, lead-lag is most likely what would be computed as optimal timing for most situations.
Roadfro - AARoads Pacific Southwest moderator since 2010, Nevada roadgeek since 1983.

agentsteel53

Mira Mesa Boulevard here in San Diego, which is an example of typical traffic-clogged arterial hell, has just about every phase possibility under the sun.  I'm going to guess that they ran Synchro or the like, because to the human observer it seems pretty random.

that said, the whole idea of moving platoons quickly?  In 5 miles, there are maybe 20 traffic lights.  I've caught all green precisely once... at 2am of course.  Other times, even as obscure as 3am, I've had the light randomly turn red with no one actually coming down the side street. 

yep, you guessed it, the one phase that Mira Mesa does not implement is flashing-yellow/flashing-red.
live from sunny San Diego.

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Brandon

Quote from: corco on December 12, 2010, 11:34:24 PM
In pretty much every city I have ever lived, traffic lights seem to flow in the following order at a standard intersection of two roughly equally important arterials where left turns are allowed with a green ball but there is an arrow

Arterial #1 Left Turns
Arterial #1 All traffic
Arterial #2 Left Turns
Arterial #2 All traffic

I just moved down to Tucson, however, and noticed that things are a bit different- the general flow is

Arterial #1 All traffic
Arterial #1 Left Turns
Arterial #2 All traffic
Arterial #2 Left Turns

The main advantage to this that I noticed is that if you're sitting in the middle of an intersection waiting for a left turn with a green ball and the light turns yellow, the next phase is going to be a protected left anyway so you're not sort of running the red light by hanging out in the intersection, and then once the protected phase is done traffic from the other direction gets to move.

I have never seen this before- do any other cities employ that sort of phasing?

It's rather common in Michigan.
"If you think this has a happy ending, you haven't been paying attention." - Ramsay Bolton

"Symbolic of his struggle against reality." - Reg

codyg1985

There is one intersection here in Huntsville (US 72/Moores Mill Rd) that as a leading left turn phase for US 72 East, followed by a through traffic phase for US 72, followed by a lagging left turn phase for US 72 East during afteroon rush hour. This is due to the amount of left turning traffic in the afternoon.

I haven't seen the lag-lag setup here in Huntsville, but I have seen the lead-lag setup used for some of the signals along University Drive/US 72.
Cody Goodman
Huntsville, AL, United States

mightyace

The greater Nashville area seems to have all three types.  (lead-lead, lead-lag, lag-lag)

However, lead-lead is by far the most common.
My Flickr Photos: http://www.flickr.com/photos/mightyace

I'm out of this F***KING PLACE!

rawmustard

Quote from: Brandon on December 13, 2010, 10:04:47 AM
It's rather common in Michigan.

I'll just add that in general, lag left phasing is fairly common with the flashing red ball/flashing yellow arrow setups and shared four-section faces while lead lefts are fairly common with shared five-section permissive faces and protected-only left-turn faces. I know I've discussed some exceptions and other quirks to this general statement elsewhere on this forum over time.

andytom

I've noticed around here that when they add FYA to a leading-left signal, they often change it to lagging-left.  This turns out being better because, if the left turn bay can be cleared on FWA, then the left turn phase doesn't need to be used at all, increasing throughput through the intersection.

--Andy

SteveG1988

There is a interesting one on NJ38 in Mt Holly. On Madison Ave, there are two jug handles, one for WB NJ38 to turn left, and the same for EB NJ38 to go right, there are two sets of lights here, it is set up for all turning traffic is in the middle of the intersection. Due to the way the intersection is set up, the barely a block apart lights go yellow sequentally, so if you are stuck in the middle you have time to go through, instead of slamming on the brakes.

http://maps.google.com/maps?hl=en&client=firefox-a&q=mt+holly+NJ&ie=UTF8&hq=&hnear=Mt+Holly,+Burlington,+New+Jersey&gl=us&ei=HJAGTfKTJYH-8Ab799jwDA&oi=geocode_result&ved=0CBgQ8gEwAA&ll=39.982299,-74.796596&spn=0.003979,0.010504&t=h&z=17

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I55,I82,I84(E&W)I88(W),I87(N),I81,I64,I74(W),I72,I57,I24,I65,I59,I12,I71,I77,I76(E&W),I70,I79,I85,I86(W),I27,I16,I97,I96,I43,I41,

Alps

Quote from: roadfro on December 13, 2010, 12:25:14 AM

What is more common in coordinated signal systems is the use of "lead-lag" (or "lag-lead") protected left turns. This is an arrangement where one direction gets the protected left turn at the beginning and the opposite direction receives a protected left at the end of the arterial phase. Lead-lag, coupled with appropriate offsets for upstream/downstream signals, provides a greater window of time to progress platoons on an arterial. If you were to use some traffic signal timing/analysis programs (i.e. Synchro) with an arterial or maybe even a grid, lead-lag is most likely what would be computed as optimal timing for most situations.
It's not common, and out of favor, because this produces the "left turn trap."  The direction with the lead left has green left arrow/green ball, then just green ball, then turns red.  Left turners are often stuck in the intersection waiting to turn left, and seeing the light turns red "frees" them to do so.  Except in reality the other direction has the lag green arrow, so they could still be coming through the intersection.  Very bad.  In order to do lead-lag, the left turns should be protected only (or at least the lead left needs to be) to avoid the trap.  Lead-lead and lag-lag both avoid this problem.

froggie

QuoteIt's not common, and out of favor, because this produces the "left turn trap."

Not if the left turn signals are protected-only (as you mention later), which is the case where I've seen use of lead-lag or lag-lead used to promote signal progression.  And that's probably what roadfro was referring to.

deathtopumpkins

Lead-lag as roadfro described is much more common here. On both VDOT-maintained and city-maintained streets usually through traffic and left turns of one direction go, then through traffic of both directions (this depends on the length of the cycle-less busy intersections omit this), then through traffic and left turns for the opposite direction. Few intersections have both left turn movements go at the same time, those that do are usually on more minor intersections that don't always have cars turning left.
Disclaimer: All posts represent my personal opinions and not those of my employer.

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rawmustard

Quote from: AlpsROADS on December 13, 2010, 07:31:56 PM
It's not common, and out of favor, because this produces the "left turn trap."  The direction with the lead left has green left arrow/green ball, then just green ball, then turns red.  Left turners are often stuck in the intersection waiting to turn left, and seeing the light turns red "frees" them to do so.  Except in reality the other direction has the lag green arrow, so they could still be coming through the intersection.  Very bad.  In order to do lead-lag, the left turns should be protected only (or at least the lead left needs to be) to avoid the trap.  Lead-lead and lag-lag both avoid this problem.

Or they can use a flashing-yellow arrow signal if they want permissive turns to be retained for a lead-lag setup (recall the "Dallas phasing" thread), although I've seen plenty of lead-lag setups where the lead is protected-only. In all honesty, protected-only lefts should be only be used in situations where it has been demonstrated permissive lefts cannot be made safely, either for traffic volume reasons or geometrical ones.

vdeane

What's wrong with a simple doghouse signal?  Perhaps it's because I'm from upstate NY, where protected-only lefts are very rare and left turns from both directions are almost always allowed to go at the same time, but I simply don't see the problem.
Please note: All comments here represent my own personal opinion and do not reflect the official position of NYSDOT or its affiliates.

froggie

QuotePerhaps it's because I'm from upstate NY, where protected-only lefts are very rare

Maybe in Rochester.  They're a bit more common in Syracuse.

Brandon

Quote from: deanej on December 14, 2010, 02:01:03 PM
What's wrong with a simple doghouse signal?  Perhaps it's because I'm from upstate NY, where protected-only lefts are very rare and left turns from both directions are almost always allowed to go at the same time, but I simply don't see the problem.

Nothing, IMHO.  I prefer a tower to a doghouse, but the concept is the same.  I think we have far too many protected only lefts.
"If you think this has a happy ending, you haven't been paying attention." - Ramsay Bolton

"Symbolic of his struggle against reality." - Reg

6a

Columbus does this (lag-lag), although I'm not sure about the rest of Ohio.  It can be a little annoying because it isn't city-wide.  One intersection will be lead-lead, while the next is lag-lag, or some combination thereof.  It really threw me for a loop the first time I encountered it.

jjakucyk

There's very few lagging lefts in the Cincinnati area that I'm aware of.  There's not many protected-only lefts either, though there's a fair amount of split phasing.  It seems that you see more lagging and protected-only lefts the farther west you go in the country.  I think part of the reason for more protected-only lefts is that there tends to be more double, or in some cases even triple, lefts in western states.  While it doesn't appear that the MUTCD requires protected-only signals at double lefts, it seems to be the generally accepted best practice.  There was one intersection not far from me where they had a left turn lane, a left/straight, and a straight-only lane (this is the right leg of a "T" intersection), and it was controlled by a doghouse.  They recently made the middle straight/left lane another straight-only one to correct that.

The question of lagging versus leading lefts is a bit harder to figure out.  There's situations where one can be better than the other.  For instance, with a short left turn pocket and high turning volume, a lagging left might cause turning vehicles to back up into the through lanes and block that through traffic until they get their arrow.  On the other hand, with a big backup of through traffic, turning vehicles might not be able to get to the left turn pocket until through traffic gets the green, and with a leading left they'd have to wait through another cycle to get their arrow.  I suspect the prevalence of leading lefts in the eastern part of the country might come from a history of having left arrows on streets that didn't or still don't have a left turn pocket.  The point is to clear out the few turning vehicles right away that might otherwise block all the rest of the traffic behind them for the whole cycle.  That really only works when one direction has an arrow and not the other, and this is in general where things get somewhat interesting.  For instance, the one lagging left I know of here in Cincinnati is just down the street from me, and it works well because opposing traffic isn't allowed to turn left at all.  It's a permissive left, so when a gap in opposing traffic opens up a number of cars can usually make the left before the arrow even lights up. 

I could go on for ages on this, but I suspect that the regional preferences for leading versus lagging and protected-only versus protected/permissive lefts probably illustrates that there's no major benefit to one over the other, just that various decisions made in the past have become standards for little more reason than that's just how it's always been done in a particular area. 

Scott5114

I would say Norman has too few protected lefts. Several of the doghouse signals go to permissive lefts when they should go protected, resulting in traffic backups as 20 or so cars want to turn left and only 3 can go because of the oncoming traffic. The ramps at the I-35/Robinson St interchange are particularly bad about this.
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florida

There are protected-left intersections down here which use both lead-lead for one direction and lag-lag for the other direction on both cross roads. Then, around 10-11pm every night, they switch them to be opposite of the daytime controls.
So many roads...so little time.

mapman1071

Quote from: corco on December 12, 2010, 11:34:24 PM
In pretty much every city I have ever lived, traffic lights seem to flow in the following order at a standard intersection of two roughly equally important arterials where left turns are allowed with a green ball but there is an arrow

Arterial #1 Left Turns
Arterial #1 All traffic
Arterial #2 Left Turns
Arterial #2 All traffic

I just moved down to Tucson, however, and noticed that things are a bit different- the general flow is

Arterial #1 All traffic
Arterial #1 Left Turns
Arterial #2 All traffic
Arterial #2 Left Turns

The main advantage to this that I noticed is that if you're sitting in the middle of an intersection waiting for a left turn with a green ball and the light turns yellow, the next phase is going to be a protected left anyway so you're not sort of running the red light by hanging out in the intersection, and then once the protected phase is done traffic from the other direction gets to move.

I have never seen this before- do any other cities employ that sort of phasing?

Scottsdale, Arizona
Goodyear, Arizona

Around Fiesta & Superstition Springs Malls Mesa, Arizona
Southern Avenue & Alma School Road, Southern Avenue & Power Road
The Pattern adds a Second Left Phase during heavy mall traffic periods (M-F after 4pm, Sat & Sun All Day, and November 20 thru January 15)

Arterial #1 Left Turns
Arterial #1 All traffic
Arterial #1 Left Turns (If Needed)
All Red
Arterial #2 Left Turns
Arterial #2 All traffic
Arterial #2 Left Turns (If Needed)

corco

Does Goodyear do that? Heh. I never noticed that.

mapman1071

Quote from: corco on January 08, 2011, 01:55:45 PM
Does Goodyear do that? Heh. I never noticed that.
I seen It At The Mcdowell/Litchfield & Van Buren/Litchfield Intersections

corco

#24
Interesting- most of my Goodyear experience is on Estrella Pkwy between Elliot and I-10, and nothing on Estrella has a lagging left- same with Cotton, and I think Dysart too (at least by I-10- but Dysart's probably in Avondale isn't it). I haven't been on Litchfield or the east-west streets much.

I noticed in Tucson today that at the intersection of Swan and Camp Lowell, the signal is a leading left and they have a sign posted indicating as much, presumably because that's a rarity around these parts. I'll get a picture of it.



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