How Long a Yellow Light Should Be

Sitting at a red light can feel torturously long, but yellow lights often seem suspiciously short. It's not all in your head: some yellow lights are too short. There is an ideal minimum length of a yellow traffic light. You just might never experience it (especially if you're from Chicago).

There's no universal correct yellow traffic light time, since people going 35 MPH will be able to stop quicker than people going 50 MPH. But there are equations to figure out the minimum safe time for a yellow light. The speed limit obviously matters, but so does how quickly a car can decelerate, and whether the road is hilly or level. Plus, you have to account for the driver's "perception reaction time," which is basically how quickly a driver can react to seeing the light turn yellow. And that reaction time can vary from person to person, which is where things get sticky.

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That's why a lot of cities are adopting equations to figure out how to set their traffic lights instead of deciding on one uniform time per speed zone. According to a 2012 survey of 200 transportation agencies in the U.S., Canada, and Germany, 40% of the agencies use an equation by the Institution of Transportation Engineers to figure out their yellow lights.

It's a much better system than just picking one time per speed zone, the current Chicago method. Yellow lights were shortened to 3 seconds in 30 MPH zones there, and results have prompted plenty of anger, especially since ticketing has skyrocketed. And an investigation from the Chicago Tribune revealed that some of Chicago's yellow lights were even shorter than the minimum of 3 seconds, and suggests city officials shortened the lights to make money off fining people for blowing them.

The Department of Transportation's traffic manual recommends that yellow lights are between 3 and 6 seconds long. Many cities err on the side of skimpy when they should be laying on the yellow for a little longer, and Chicago is an example of a straight-up bad yellow light policy.

The ITE's equation is an improvement, but there may be an even more nuanced approach transportation agencies could take. Heshem Rakha, a transportation expert and engineering professor at Virginia Tech, headed up a project to figure out a better standard that takes into account the fact that some people just aren't going to react as quickly as others.

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"Basically, when you're designing a yellow time, you want to prevent bringing the drivers into what we call the dilemma zone, where you have no good decision you can make," he told me over the phone. Yellow lights that are too short put drivers in the dilemma zone when they get stuck in the intersection or ram their car to a halt and cause a collision. Which is why Rakha's equation tries to pinpoint a more precise (and often, much more generous) length of the yellow light. The dilemma zone is shitty and scary.

Rakha's proposed standard is more flexible than the ITE's, but at the bare minimum with current traffic standards in place, a yellow light in a zone where people are driving 35 MPH should be at least 4 seconds. Chicago's 30 MPH yellow lights last exactly 3 seconds now. For cars to clear an intersection at 99.6% at 35 MPH on Rakha's chart (on a 0% grade, which is what Chicago's flat streets would be on) a light should be 4.9 seconds. Yes, Chicago's speed limit is slightly lower, but come on. The recommended absolute minimum yellow light is 3 seconds. This minimum is not safe or appropriate.

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When drivers approach an intersection as the light turns yellow, some people slam the brakes, others race the red. The wrong split-second decision causes accidents and racks up expensive tickets—sometimes far more than necessary. And this is partly because many cities do not allow long enough yellow lights.

This is some bullshit.

Longer yellow lights would give people more time to make it across the intersection without gunning it or slamming on the brakes or racking up a giant fine. Even the standards set by Rakha, which allows for a variety of reaction times, do not take into account distracted driving, speeding, or just plain asshole roadrage. If yellow light times were a little more generous, yes, it would slightly slow the flow of traffic, and cities wouldn't be able to collect as much money from tickets. But too bad, cities! Suck it up and be safer.

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Image via Ruthanne Reid/Flickr (CC BY 2.0)

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