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 85^th 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. [Read more…]