Thursday, December 1, 2011

What Does a Yellow Light Mean?

It's not a trick question. Stop and think about it before reading further. Unfair Park's Jim Schutze, normally a surprisingly level-headed curmudgeon despite the photo of him that accompanies his blog showing him aiming some kind of rifle vaguely in the direction of the reader, asks that question and answers, no less than four times, that a yellow light means you're supposed to slow down.

After the jump, why you should stay away from Jim Schutze when he's behind the wheel.

First, the right answer to the question. Unfair Park reader "Hannibal_Lecter" tells Schutze in the comments:
While a flashing yellow light means caution, a solid yellow (technically amber) light does not mean slow down. When used in a traffic signal, under Texas law all a yellow light indicates is that the signal is about to change to red. You do not have to slow down for a yellow light.
Source: Unfair Park.

It's a judgment call. If you can safely stop before the light turns red, you should slow down on yellow, as Schutze says, and stop. But if you are too near the intersection to safely stop, you should continue on through the intersection. You can slow down, but sometimes maintaining a steady speed is safer. More accidents are caused when one driver makes a sudden change in speed than when all drivers maintain a steady speed. The one reaction that is almost always wrong is to speed up on yellow, because changing speed is unsafe. And speeding through intersections is unsafe even on green.

Now, this blog post isn't meant to be a driver's test. It's meant to remind us that some things that we take for granted, that we think we know, that we think everyone else should know but doesn't, sometimes aren't quite what we think they are. Knowing the rules of the road is only one example. It's called illusory superiority, above average effect, superiority bias, the Lake Wobegon effect. Almost everyone, not just Schutze, thinks they are better drivers than the others they share the road with. One study showed that 93% of Americans believe they have above average driving skills.

So, back to my tease just before the jump. Why should you stay away from Jim Schutze when he's behind the wheel? It's nothing personal. You should stay away from *everyone* when they are behind the wheel. And stay away from bicyclists, too, especially kids on bikes. And parked cars, too, for that matter. And curbs and telephone poles and fire hydrants. They are all out to get you, even the inanimate objects. Repeat, they are *all* out to get you. Drive like that's true and you'll lessen your chances of having an accident. When driving, paranoia is not a mental disorder. It's a survival skill.

P.S. If you think solid yellow lights are confusing, the flashing yellow arrows can be even more confusing, especially in Richardson, as the city is only part way through its conversion to a flashing yellow arrow for an unprotected left turn. (Read this article from 2009 for more than you'll ever want to know about left turns.) When I first encountered the flashing yellow arrow, it struck me as odd and slightly dangerous. Now, fully familiar with the flashing yellow arrow, when I come to an old style intersection with the solid green unprotected left turn signal, *that* strikes me as really dangerous. Luckily, I'm an above average driver.


Elias said...

Get rid of traffic lights. Make the streets two lanes and make every intersection a 2 or 4-way stop and you'd have much fewer accidents. Or you could do this:,1518,448747,00.html

Mark Steger said...

Elias, thanks for the feedback. You probably would have liked my earlier post, "Imagine There's No Central."

Brian said...

Mark, amber light does not mean slow down. If you decide to go, the amber light formula requires that you proceed to and into the intersection at the speed limit or more. That is the meaning of the formula, and the definition in the 1984 Transportation and Traffic Engineerng Handbook.

See for the physics

Brian said...

Mark, one cannot slow down when they see a yellow light at a signalized intersection. Yellow light either means "stop" or it means "proceed to and enter the intersection going the speed limit or more." That is meaning of the physics in the federal formula traffic engineers use to set yellow lights, and the verbal definition from the 1984 Transporation and Traffic Engineering Handbook. I placed the math in the papers on the homepage at

Mark Steger said...

Brian, if you can safely slow down and stop, that is a good practice. The question is what to do when one can't stop safely. Then one should proceed through the intersection. The most danger arises when a driver *changes* speed. So, proceeding at the same speed is the recommended procedure when you can't stop safely. Speeding up or slowing down as you cross the intersection is what you should do if proceeding at the same speed as before is unsafe. In that case, whether you should speed up or slow down depends on circumstances.

Mark Steger said...

It may or may not be original, but a T-shirt for sale in Hanoi has these rules of the road there:

Green - you can go.
Yellow - you can go.
Red - you can still go.