Ever wonder how speed limits are set?

CABO » Ask the traffic engineer: How are speed limits set?

Just had a discussion with a friend about speed limits. I told him a summary of what the traffic engineer says at the link above.

Most people think that motorists base their speeds on the speed limit. But while that may be true on freeways and other high speed highways, it’s not true on lower speed roads and streets. On lower speed roads and streets, motorists base their speeds not on the speed limit but on how the road feels.

If you’re typical, that doesn’t feel right to you. You may be thinking to yourself, doesn’t everyone pay attention to the speed limit like I do? Sure, there are some people who do, but observations over the past 60+ years of motorist behavior in response to a change in the speed limit shows that on roads and streets that are not subject to a statewide maximum speed limit, the change in actual speeds is often too small to measure. I learned early in my career as a traffic engineer not to project my own beliefs and behaviors on the rest of the population. Response to speed limits is just one example of that.

Since speed limits do not have a measurable influence on actual speeds, it makes sense to set speed limits so as not to make lawbreakers out of a large fraction of motorists. Speed limits that are set too low makes law enforcement more difficult and leads to a disrespect for speed limits.

Shown here is a speed distribution on a rural road. Notice that the distribution is quite narrow, meaning that most motorists drive at nearly the same speed. You will see that changing the speed limit by just 5 mph will make a big change in the number of violators.

So the 85th percentile rule is simply a recognition that speed limits are set for the purposes of enforcement. And since law enforcement only has the resources to cite the most egregious violations, the speed limit is established at or near the 85th percentile, making only about 15% of free-flowing motorists violators.

The author, Bob Shanteau, further down in the article, serves up two very interesting details, along with the above point that drivers do not change their speeds when the speed limit changes.

One, that many policemen have told him over the years that the worst speeders are politicians.

Two, that those who drive in the 85th to 90th percentile are the safest drivers, and those who drive over the 90th are the worst..

Somebody tell that to the guy who started the practice of setting it at 85%. Aww, heck, never mind, because he said they never give out tickets for five to ten miles per hour over.


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