Many people ask if yellow traffic lights are long enough. Either they are involved in an auto accident or they have narrowly escaped an accident. We expect a yellow traffic light to last long enough to safely clear an intersection without an accident. Sometimes we think yellow lights are too short or too long. Many of us do not know much about how yellow light changes are timed.
You have probably noticed that the yellow light lasts longer at intersections on busy roads or where speed limits are higher. The timing of the change from yellow to red is called a yellow change interval. The length of the interval has been questioned repeatedly in the context of red light cameras that are not correctly timed. Most U.S. yellow change intervals are calculated with a formula originally created in 1965. The formula had several key elements: Perception Response Time (the amount of time necessary for a driver to see a traffic light and react when the light turns yellow), the 85th percentile of approach speed in miles per hour, and a pre-determined deceleration rate. The formula was revised in 1982 to add the approach grade of the road.
In August, a panel of experts from the Institute of Transportation Engineers met in response to an appeal filed on behalf of a group of engineers who argued that the formula is flawed and that it cannot be applied without modification to every intersection in the nation. After due consideration, the panel agreed that best practices in use now may not be adequate for drivers who are making turns. Their conclusion was that a “one-size-fits-all” formula does not make intersections safe.
The goal of yellow light timing is to prevent forcing drivers into a “dilemma zone.” The dilemma zone offers two options to the driver in the intersection, both of which are bad choices: to speed up to get through the intersection before the light turns red or to slam on the brakes and cause a crash. Many proposals offered recognize that drivers are making a split-second decision in an intersection and extend the timing of the yellow light.
Another factor in lengthening yellow light intervals is the additional time needed by trucks or by vans or SUVs carrying a heavy load. Not only does the weight dictate additional time to stop, but it also creates greater danger if there is a crash due to the rollover potential of the vehicles.
The issue in establishing new yellow light intervals is not accommodating red light cameras but making intersections safer for all drivers. Slowing the flow of traffic minimally or being overcautious in setting intervals seems far preferable to creating intersections that are more dangerous. One question proposed new calculations may need to consider is the additional time needed for an impaired or distracted driver to recognize and navigate an intersection.
Are yellow traffic lights long enough to keep us safe? Hopefully, the engineers and others responsible for developing more workable equations will be able to make the roads safer and prevent more accidents in intersections. Hopefully, new equations will save lives.
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