Lagging Left-Turn Arrows

by Nick Coons
Dec 3, 2003

Chandler and Gilbert are currently conducting a study to find out the advantages and disadvantages of using lagging left-turn arrows instead of leading left-turn arrows. Leading arrows change green before the through-lights are green. Lagging arrows change green after the through-lights' green phase has completed.

Leading arrows are commonly found throughout the valley. Most of us here are accustomed to having green arrows to make a left turn before the through-traffic receives their green lights. However, in Scottsdale and similar areas (like Fountain Hills), lagging arrows have been in use for quite some time.

Intersections along McQueen Road, including Elliot, Warner, Ray, and Chandler, have all be switched to lagging arrows to assist Chandler and Gilbert in their studies.

Theoretically, the through-lights are supposed to be timed. This means that if you go through an intersection during a green light, and you travel the speed limit, or just under, that you will also hit a green light at the next intersection. And I say "theoretically" because some streets have their timing set very well, and others do not. You must realize, however, that timing is such a difficult task, because to time each intersection this way, it also affects every other intersection around it and its timing. In some places, it may not even be possible.

Let's say, for instance, that all intersections' green lights are timed perfectly. Then there would be a definate advantage in travel time to the driver with lagging arrows. Imagine that you are at an intersection with leading left-turn arrows, and you are awaiting your green light. Your green light will come just after the cross-traffic light has turned red. That means that you will be at the tail-end of cross-traffic once you make your turn, and you will likely miss the green light at the next intersection, having to wait an entire cycle to make the green light.

With lagging left-turn arrows, this is eliminated. Because lagging arrows cause you to receive a green left-arrow before cross-traffic gets their green light, this puts you at the head of the cross-traffic pack and means that at best you will catch the green light, and at worst you will have to wait for five seconds for a green light because you arrived to quickly. This is better than having to wait an entire cycle.

But in real-life, all intersections are not timed properly, and even if they are, traffic conditions determine real-world speed more than the posted speed limit does, so it wouldn't necessarily work as planned anyway. Aside from that, lagging arrows are a safety benefit.

When an intersection has passive left-turns (this means there are no red left-turn lights), one often pulls into the middle of the intersection when there is not a green arrow by there is a green through-traffic light. They sit and wait for traffic to clear, and sometimes this means the driver has to wait for a red arrow. By the time the driver has a chance to complete their turn, the cross-traffic has already received their green light.

With lagging arrows, this poor soul in the middle of the intersection would be able to complete his turn with the safety of a green left-turn arrow, and knowing that he can complete the turn without the cross-traffic's light turning green.

Lagging left-turn arrows present a definite benefit, and should be implemented throughout the valley.