Stop me if you've been this person... you're driving down a busy street, you know you're about to come up on a busy intersection with a stoplight that feels like an eternity when you miss the green. Then, as you approach, you can see it in the distance... green turns to yellow. Your heart suddenly begins to race. Your right foot has a split second to decide, punch it, or start pushing down on that break...

What do you do? PUNCH IT, right? You do... and you MISS the light! "WHAT?!" You shout, "How long was that yellow? Two seconds?!" I know I've been that guy, and to answer that question, nope. It was not two seconds. No yellow light is two seconds as I learned from John Witt, the Traffic Engineering Program Manager for Cedar Rapids. Actually, I learned a lot more about stoplights too.

Here's how the length of yellow stoplights are determined

As mentioned, I reached out to the good folks at the Cedar Rapids Public Works. After a short amount of time, John Witt gave me the answer on stoplights, traffic lights as he calls 'em, that I was looking for about yellow lights.

What he told me was the length of a yellow light, which he called 'the yellow clearance interval' actually ranges between three seconds and six seconds. But how is that three-second window decided on?

This a three-second light? Should I stop, or go?
Photo by David Guenther on Unsplash

Mr. Witt went on to explain that it depends on the location and the posted speed limit for that road. The higher the speed on a roadway, the longer the yellow light will be prior to switching to red.

Oh, and the REAL purpose of a yellow light stoplight? It's not for stragglers to go flying through. It's to allow vehicles that are near the intersection to stop or safely clear the intersection before folks waiting at a red get the green light. The keyword John Witt used is 'safely'. Meaning, we're not supposed to go 10 miles per hour over the speed limit to make a yellow light.

How about the length of a green/red traffic light?

There's a little more technology, and less science behind many, but not all red/green intervals. It has nothing to do with a stoplight detecting weight either. On most busy streets, a light will change to red when electromagnetic induction detects that there is traffic waiting at the intersection. This according to Rhythm Engineering. In other words, there's really not a set length for the green or red time on a normal traffic light.

So, please drive safe! And remember, that yellow light is not only two seconds long. Promise.

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