Traffic Light Logic

During the summer of 2001, I worked at the Air Liquide corporate offices in Houston, TX, as a lowly intern. Besides sneaking out of work every day at 2:30 pm to go play golf, one of the things I remember most about my first experience in Texas is the way the traffic lights were configured. Weird, I know, but allow me to explain.

I’ve spent a considerable amount of time in both Louisville, KY, and Atlanta, GA, and the traffic lights work the same in both places. Specifically:

  • Lights are hung vertically
  • Most important, left turn signals turn green BEFORE the lights that allow cars to go straight through a particular intersection

In Texas, however, traffic lights are different, at least in urban markets like Houston and Austin. Instead of vertical lights, the lights are hung horizontally. More important, however, is the fact that the left turn signals go off AFTER the straight traffic has been allowed to proceed through the intersection.

When I first noticed this difference in traffic light configuration back in 2001, I thought about it for a second and decided there was no real advantage to either setup. I just figured that this was how they did things in Texas, because – you know – Texans want to be different.

When I arrived in Austin for this year’s SXSWi and hopped in the rental car, I immediately encountered these silly horizontal traffic lights, and that got me thinking about the subject once again. I hadn’t made a return trip to Texas since 2002, and an interesting thing had happened to me in the meantime.

In 2003, I got in a terrible accident in Atlanta. Basically, a random dude was waiting to take an unprotected left hand turn (without the green turn arrow) across three lanes of traffic at a busy downtown intersection (you’ve seen these people – they try and make their turn after the light has turned red, and they do so because they’re stuck out in the middle of the intersection). I was going straight through the intersection and had gunned it to try and make it through the yellow light. Because of the yellow light, the rest of the straight traffic had begun to stop, and the guy making the unprotected left hand turn decided to sneak across three lanes of traffic because he was blocking the middle of the intersection. I was going about 50 miles an hour, and this guy seemingly popped up out of nowhere and was now directly in front of my car. With a stab of the brakes, a miracle, and some good slalom driving, I managed to guide the car through a sea of street signs, traffic poles, telephone poles, and store marquees, and I only hit a tension wire that was attached to a telephone pole. I walked away from the crash without so much as a scratch. My car was totaled.

So, fast forward to Austin in 2006, and I’m sitting here looking at these silly horizontal traffic lights, waiting for my left arrow to turn green so I could get rolling. Of course, this got me thinking, “Why the hell do the turn arrows go AFTER the straight green lights?”

Then it hit me.

SAFETY! With engineering and logistics, the most common reason for an otherwise inexplicable method of operation is a safety underpinning, and I believe this is precisely why the Texas traffic lights are configured thusly.

Think about it: if the turn arrow is the last to go, then people aren’t going to be planted out in the middle of the intersection waiting for their chance to gun it across three lanes of traffic, just so they won’t have to wait through another light cycle. Although I haven’t done any research on the topic, I’ll bet good money that unprotected left hand turns are the number one cause of accidents in typical intersections. People make idiotic decisions in the name of avoiding another light cycle, and gunning it after the light has turned red is pretty tempting in our hustle-and-bustle world.

Given my accident history, I immediately became fascinated with this idea. In fact, I couldn’t go to sleep last night because I was thinking about it so much. If you can get over the fact that I’m a total geek for that, you might actually identify with my thinking here. I could have gotten seriously injured (or worse) that day in Atlanta, and I absolutely believe that my decision-making would have been completely different if the traffic lights there mirrored those in Texas. First of all, circumstances would not have been the same, and second, other people’s driving habits change because of the nature of the system. Everybody is forced to make more rational, sane driving decisions because they aren’t given this opportunity to whip it across traffic in order to make a left hand turn.

I think it’s genius.

In fact, I think it’s so genius that I’d like to see every major US city adopt this traffic light configuration. Not changing is sheer negligence, in my opinion. Kentucky, my home state, runs a program on the TRIMARC signs around Louisville that shows the number of deaths on Kentucky roads each year. Between 2004 and 2005 (the only two years for which I have quick data), there was no progress in this area. I think a logistical change on traffic light configuration would make a significant dent in the number of deaths in Kentucky each year, despite the fact that it’s basically a rural state. In Atlanta, the impact would be far more noticeable, as traffic there is not only more dense, but also more hectic.

I think maybe I need to make a public policy push for this in eastern US markets. I suppose that because it affected me personally, I’m probably more inclined to champion this cause. Regardless, I’m going to see what I can do about this. Safety is important, and accidents are just a pain in the ass!

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32 comments… read them below or add one

Devin March 13, 2006

Depending on the time of day some of them here will do the left turn signal after or before. It just depends on how it wants the traffic to flow and in which direction. Interesting post though… geek.


Anonymous March 15, 2006

who was doing the driving and why? ha ha. you forgot to tell them that. blllllllllll


Jennifer Grucza March 16, 2006

For busy streets with no left turn arrow, sometimes the yellow light is your only chance to make a left turn.

Of course, then there’s Boston, where left turners often dart out first, before oncoming traffic. That really took me by surprise when I started driving here.

Another thing about Boston is there are these delayed greens, where you’re supposed to someone magically know that you can turn left because the other direction is still red. They don’t bother to add a green arrow. So basically you just have to memorize which intersections these are, and keep a watch on oncoming traffic to see when they start moving.


tfro March 26, 2006

well, luckily you were OK in that little incident. But it led you to see the light in cars…


Chris P. March 26, 2006

Long live the Bimmer!


Chris June 28, 2006

I, too, think about traffic patterns in my spare time; mostly while sitting in traffic. I just moved to Pittsburgh where almost every intersection is adorned with a dedicated left-turn lane. Initially I thought this was odd, but a little deduction revealed that this configuration makes much more sense than the dedicated right-turn lane. Since you can almost always make a right on red, the right lane almost always flows better than the left; there will always be people waiting to make a left-hand turn against oncoming traffic.

The dedicated left-turn lane segregates the traffic that normally holds up unawares straight-ahead drivers, thus reducing the number of impatient people who make ill-conceived jumps out of the left lane (that is stalled) into the free-flowing right-lane. Used in conjunction with the Texas delayed turn I think we might be onto something!


Neuman November 26, 2006

“you’ve seen these people – they try and make their turn after the light has turned red, and they do so because they’re stuck out in the middle of the intersection” — quote from you.

“If you enter an intersection while the light is green, you may finish your turn even though the light turns red.” — quote from traffic manual.

” I was going straight through the intersection and had gunned it to try and make it through the yellow light.” — you.

“People make idiotic decisions in the name of avoiding another light cycle, and gunning it after the light has turned red is pretty tempting in our hustle-and-bustle world.” — you.

Now, given these quoted I think you are the idiotic and hypocritical driver who’s too caught up in getting through the light to not waste another cycle. As I almost got hit by one of you idiots… learn how to drive, and ffs stop on yellows.


Chris P. November 26, 2006


I know you think you’re slick and caught me in some kind of contradictory statement, but the entire reason I wrote this post was to place an emphasis on safety.

I said that people make idiotic decisions to avoid light cycles, and I was acknowledging the fact that I had made a poor decision that resulted in a terrible accident.

By writing this, I had hoped to clue people into some of the dangers here, and I also think that there is simply a better, safer way to construct traffic light cycles.


Andrew February 21, 2007

The simple answer is build a roundabout – they reduce fatal accidents at intersections by up to 90% and improve traffic flow.


Luke April 25, 2007

Problem is, people generally hate roundabouts…I know I do. I probably shouldn’t have moved to Massachusetts.


Avent September 26, 2007

Appreciated your story and like everyone I contemplated the same problems while setting in traffic. I was wondering who the police blamed for your accident. From where I sit, both drivers were equally at fault, bad judgement on both sides. I know this is old saying but if you are going make an error, “ere’ on the side of caution”. Hmmm..could this be called “defensive driving”.

This is a terrible traffic problem that needs to be address and at the least some kind of continuity across the nation. Red states>horizontal, Blue states >Vertical????????? HehHeh…….


Robert November 17, 2007

Sadly, in Michigan, there is great inconsistency in how the left-turn lights are timed, usually involving different municipalities (although some municipalities have both!). I do prefer the latter, which seems to be more prevalent.

Newer left-turn signals in Michigan, though, installed in the last couple of years, are confusing, though, because they have two yellow left arrows. I looked up signals in the MUTCD, and this configuration is not supported. I just think it’s stupid. (Older left-turn signals in Michigan have a “LEFT” sign, with red and yellow circles and a green arrow; the new signals are all arrows.)

The problem with roundabouts is the thinking that people who want to go straight, *want to go straight through*. They don’t want to have to stop and make a looparound.


Anonymous January 5, 2008

When you are sitting at a green light with no left turn arrow, you are supposed to move into the intersection and wait for the light to change or for a space in the oncoming traffic to make the left turn. If there is no space in the oncoming traffic you are supposed to wait for the light to change to red and the oncoming traffic to stop to make the left turn. The law allows for 2 vehicles to make the left turn at this point. You must be alert and quick to make the left turn before the cross traffic starts to move.


Dave March 23, 2008

How about an article on Wordpress safety? Lots of folks are getting hacked, and you’re a big voice out there bringing people to Wordpress. Security is a big issue, and it’s never going to get smaller. I couldn’t find an article on your blog (show me if I missed it) about this.


Kev July 9, 2008

What you’ve described is known among traffic engineers as the “Yellow-Trap” problem. Solved by left turn signal timing as you’ve observed. Check out this page for more traffic geekiness.


Bry July 17, 2008

I got into the same situation the other day with making a left turn on a yellow light. My car is now gone. I believe sometimes it can be the drivers fault who is driving pass the yellow light. I learned in my drivers ed class: “Yellow means prepare to stop, or slow down!” As I was turning the driver failed to slow down while all the other cars have and she hit me! Isn’t it against the law to run or speed up to a yellow light? I guess everyone should speed when they see yellow.


Mike K November 11, 2008

I know what you’re talking about with the late cycle left turn arrows.

I live in Denver and only a few intersections have this (most are still early cycle left turns before the straight traffic goes across) but it’s starting to catch on, especially in the suburbs. Some intersections have it where everyone on one side gets the straight and the left turn, then the otherside can go straight and then later the other side gets the left turn and that other side turns red at the same time.

As far as horizontal signals go, I’ve heard so many reasons why some states do this. I’ve heard “to be different” all the way up to “reducing down on wind resistance that could potentially damage the light.” I’m thinking it’s moreso to be different because we get a LOT of wind in Denver and all of our signals are vertical. There is ONE intersection where it’s horizontal and it’s in the middle of a large office park in a pretty well-off part of the metro area.

Chicago gets a lot of wind too and all of theirs are vertical.


Will June 9, 2009

As a traffic engineer in Texas, I can confidently say that most urban signals are configured for efficiency as well as safety. Of course, if an intersection runs efficiently it tends to be more safe because drivers do not become frustrated with stale greens and long waits. You should read up on “Dallas Phasing” and the yellow trap… we run many intersections where one side receives a protected arrow at the beginning of the cycle while the other direction receives it at the end. As for horizontally mounted signal heads, it’s simply preference. We mount ours horizontally because it looks nicer.


Ben July 21, 2009

I grew up in Tucson, AZ, which has left turn arrows after green, and it’s by far the most efficient method I’ve seen… now that I live in Austin, the arrow-before-green only allows the people at the front of the line to turn, rather than allowing the left hand turn lane to fill up as through traffic goes by. Frustrating as all get out.


B T Manning December 14, 2009

Turn if you must. In the U.S,A I think red is on the left. In Japan red
is on the right. In Canada the red is on both ends. My color-blind friends
have it tough if you mix it up.


Eris December 30, 2009

I wouldn’t normally bother commenting on a three year old post, but coincidences and the internet pretty much forced my hand. I was on a Google quest for the exact rules on turning right at a red light with a green arrow because I recently got a ticket. The coincidence? The intersection was at Westheimer and 610; I was on my way to work at Air Liquide!


Chris Pearson December 31, 2009

Eris — That’s wild! The Internet is a crazy, crazy place :D


Mike K January 1, 2010

@ BT Manning,

Not all signals in Canada have the double red on one signal. In fact I think that’s only for Quebec.

My wife and I vacationed in Victoria and Vancouver, BC recently and the signals are just like the US except the 12″green and yellow with 18″ red are more common than the 18″ for all three. The only thing that got me about lights there were the flashing greens at pedestrian crosswalks; such as in front of schools. Oh and the green left turn arrow will also flash if it is going to become a regular left turn yield.

Thing is about driving in Vancouver is that there are very few devoted left turn lanes and most streets are rather narrow! The only time you see them are at the very big intersections in the suburbs. Otherwise, if you’re driving in the left lane on a regular street and don’t want to turn left, odds are you’ll get stuck behind someone who does.


Danni February 7, 2010

Wow…. this makes a boatload of sense to me. I recently moved from Orlando, Florida, where the turning traffic goes first, to Virginia Beach, Virginia, Where straight traffic goes first. It’s been very hard to get used to and I couldn’t figure out for the life of me why the hell it would be like that but now it makes perfect sense. Chris, you’re awesome! Maybe now I’ll start appreciating it instead of bitching about it…


Jennifer Whiteside April 17, 2010

They type of signalization that you are referring to that almost caused our accident is a protected/permitted signal . It allows people to turn left when the through lanes have green lights. Since people are able to turn left without a protected traffic signal having them turn left later doesn’t really help all that much because they are still going to be trying to inch through the intersection when the light is green. There are sight distance issues. That’s why some intersections are protected/permitted and some are just protected. The only way to avoid all of these accidents is to make all lights protected.

Also, those lights are about 4 ft tall, so when you turn them horizontally, the structure holding them up doesn’t need to be as tall to maintain the same vertical clearance. Also, the area subjected to wind (in something like a hurricane) is less so they don’t have to be as stout either. It saves money.

You can’t change all the signals in a locale anyway because of driver expectancy… drivers expect the same thing to happen at all traffic lights (and in all traffic situations) every time. If you change all the traffic signals, people gun it when they see a green light even if the light isn’t green for their particular movement. It would cause all kinds of accidents.


Scott May 29, 2010

“As for horizontally mounted signal heads, it’s simply preference. We mount ours horizontally because it looks nicer.”

Perhaps you should reconsider. I’ve discussed this issue with two color-blind Texans. They can tell whether it’s the left or the right side that is lit, but when it’s late in the day and they’re a little tired, it’s hard to remember whether it’s red on left and green on right, or the other way around. Nobody confuses top and bottom, but confusing left and right is very common. For the sake of safety, you shouldn’t rely on a signal that 3 or 4 percent of the population cannot read and have to memorize.


Frix July 16, 2010

this post reminded me of the poem that i memorize as a contest piece when i was in grade two: “Stop and Go. When you go down the street, use your eyes, then your feet. Boy and girls you should know, red is stop and green is go.”


Sam March 11, 2011

Here in Nebraska, horizontal traffic lights are used all over the state, except for Omaha.


matter April 6, 2011


The accident you describe was clearly your fault. When one approaches a yellow light, one should stop (if one can stop safely) not “gun it” to try to blast through the intersection. Furthermore, a vehicle which is already in the intersection has the right of way. That is why people who KNOW how to turn left enter the intersection on a green light in the first place… to gain the right of way, so that they can complete their turn when the light turns red.


Jim Sonnenberg January 15, 2013

Hopefully you were cited in that accident. And while you were proud to have maneuvered to save yourself, you neglected to mention the fate of the other driver. I mean, you were going 50 on a congested downtown street and you GUNNED IT!? If the poor bastard suffered injuries I hope he sued you. From your description you hit him hard.

Btw horizontal signals are everywhere. They’re not as unusual as you think. You’ll find them in downtown Cincinnati, Tampa and even some streets in downtown Chicago.


zebby April 8, 2013

You “gunned it” through a yellow? That’s safe. Also, if your light was green for that long (that it was now turning yellow) no one at all should have been in the intersection ie: the guy you had to avoid in the intersection, or all the other cars headed in the same direction as you would have collided with him too. The filter light should go off first. Not last. Never heard of that.


Averill Hecht October 28, 2013

I love Quebec’s and PEI’s Symbolic shape coded traffic signals. And have these lenses in my collection. I also have video of these which I have filmed in Quebec. I’m surprised that associations for the blind haven’t pushed these signal lenses in the US for colour blind drivers under the ADA. I really thought they would take hold in Canada. I know British Columbia was thinking of using them for a while. PEI does use them. They were used in New Brunswick and Nova Scotia, but never made it to wide spread use. They have since been replaced with standard signals in those provinces. Also, Quebec stopped using them on provincially maintained highways, but is still legal, there. And still maintained in some municipalities. I have these provincial standards. By the way, these signals are placed in the horizontal position, which you show, and, vertical.


Hoot and/or Holler

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