By Wilfrid Nixon
Every January, the Transportation Research Board (part of the National Academies) holds an annual meeting. One of the presentations I heard this year discussed whether safety and mobility on our roads would improve if the yellow-light time in traffic light systems were lengthened during rain events, and if so, how much that yellow light interval should be changed.
It turns out, this research paper is pertinent for the current discussion on traffic cameras in the Iowa Legislature.
Determining the length of time for the lights in traffic signals involves various parameters, two of which are perception time and reaction time. Perception time is how long it takes us to notice something (in this case, how long it takes us to notice that the lights have changed from green to yellow). Reaction time is how long it takes us to act on the change we have just perceived.
All of us have different perception and reaction times, and those times may change depending on the time of day, how much coffee we have had, whether the kids are screaming in the back, etc. Unfortunately, traffic light systems cannot yet take all those factors into consideration, so their timing is designed according to standard perception and reaction times.
One of the things traffic engineers strive to avoid when designing signal timings is the creation of a “dilemma zone.” This occurs if a driver, even though following the speed limit, can neither get through the light before it turns red, nor stop before the stop line at the intersection. Some people, in general the elderly, have slow perception and reaction times. They may do all the right things, but, through no fault of their own, get ticketed by a camera system.
During rain events, the size of the dilemma zone increases as the system doesn’t account for the weather.
Of course, this is not the only argument against red light cameras. Many studies have shown that if you want to improve mobility and safety at a signalized intersection, you should increase the time of the yellow-light phase. If this time threatens to become too long, then an “all red” phase (where all lights at the intersection are red) can be used.
We should not allow red-light cameras in Iowa, but if we do, they should be allowed only after the timing of the lights have been adjusted to a longer yellow period, and a study explores whether such an adjustment has improved safety. A red-light system would be approved only if no safety improvement had been observed.
I suspect under such a system we would have no need for red-light cameras.Wilfrid Nixon is Professor of Civil and Environmental Engineering at the University of Iowa. Comments: email@example.com