The Science and Safety of Speed Limits

A 13-mph speed limit sign

In the past week or so I’ve driven past two speed limit signs that particularly caught my attention. I was able to get a photo of one of them (pictured above.) Not 10 mph or even 15 mph. 13.

That’s odd, I said aloud to myself.

Then I got to wondering … Is the speed limit set 13 mph because of speed and safety risks (acceptable at 10 mph, but too dangerous at 15 mph), or because of the psychology of using an uncommon number. Is the posted speed limit set to a different number to catch our attention and, thereby encourage us to be mindful of our speed?

When I first noticed that 13 mph sign, I wasn’t sure if it is was more physics or psychology. I don’t know enough about the science of car crashes and bodily injury.

The following day, I noticed a 14 mph speed limit sign posted only a mile or two away from the 13 mph sign. Hmm … that makes me think that the chosen speed limits are more psychology than physics.

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