How Long Should a Yellow Light Be?

If you've ever driven a car in a place that has installed red-light camera technology for traffic enforcement, you've probably noticed that the yellow lights at the intersections with those cameras are a lot shorter in duration than they are at non-monitored intersections.

The reason why is pretty straightforward. The state and local governments that generate revenue from issuing traffic tickets for people who violate red lights benefit if they can trap more drivers into entering an intersection after the traffic signal changes from yellow to red. And a very easy way to do that is to set the duration of the yellow caution signal at red-light camera intersections to be shorter than at other intersections, which makes it more difficult for drivers to anticipate how long they will have before the light may turn red.

But how long should a yellow light really be if what we really want is safe intersections? We've taken the math developed by the Institute of Traffic Engineers (ITE) and build the tool below to do it for you. You just need to enter the data that applies for your intersection of interest, and we'll do the rest!

(If you're reading this article on a site that republishes our RSS news feed, click here to access a working version of this tool!

Driving Data
Input Data	Values
Velocity of Vehicles Approaching Intersection [miles per hour] - The 85th Percentile Velocity for traffic approaching the intersection, if known, or a speed that is greater than or equal to the Posted Speed Limit.)
Reaction Time to Brake [seconds] - The time needed to note the change in the traffic signal from Green to Yellow and to step on the brakes.)
"Safe and Comfortable" Deceleration Rate [ft/s²]
Road Grade [%] - The grade or slope of the road approaching the intersection (positive if uphill, negative if downhill.)

How Long Should a Yellow Light Be?
Calculated Results	Values
Minimum Yellow Light Duration [seconds]

In the tool above, we've entered the median values measured for both the Reaction Time to Brake and the "Safe and Comfortable" Deceleration Rate, which are in close agreement with the values recommended for use by the Institute of Traffic Engineers.

Our tool then provides the minimum duration that a traffic signal should be yellow before switching to be red. If the traffic signal at your intersection of interest has a notably shorter yellow light, that's a pretty good indication that you might be dealing with a red light ticket trap.

Not to mention a potential traffic hazard, put in place by a government bureaucracy that cares more about getting money from traffic tickets than they do about the safety of the public.