There’s a grumbling around the nation. Wyoming lawmakers want to know why the speed limit is so low on roads in the middle of nowhere. A Massachusetts politician asked why a road with a 30 mile per hour speed limit was designed for 40 mph traffic. Michigan State Police have succeeded in eliminating many Interstate speed traps, but they haven’t been able to fix the 55 mph zones in Detroit.
They are victims of the American tendency to make numbers more important than the things they represent.
More specifically, they are victims of the unfortunate choice to use “miles per hour” in both highway design and speed enforcement.
Design speed is forward-looking, the answer to the question “how fast do we expect drivers to go?”
Speed limit is retrospective, the answer to the question “how fast are drivers going?”
Trying to equate them leads to trouble because they are inherently different.
The formulas in the highway design book aren’t very good predictors of actual behavior. I’ll write more on this another time, but part of the problem is states can save money by using a design speed slower than they know traffic will go.
And here come the lawyers, from both directions.
On one side, DOT CYA. To quote a Texas Transportation Institute report, “liability concerns arise even though drivers can safely exceed the design speed.” Translated into English, giving undeserved speeding tickets makes bureaucrats sleep better.
On the other side, you have environmental groups. Long before you hear about a road improvement project they are already preparing a lawsuit to stop it, and they may demand a low speed limit to settle the suit.
The correct response is to such demands is, “Federal law does not allow setting a speed limit without a measurement of the actual speed of free-flowing traffic. See Manual on Uniform Traffic Control Devices section 2B.13.”
Think anybody in court cares about speed traps? DOT lawyers’ job is to get construction money released by making the lawsuit go away, and obeying the law is an obstacle to that mission.
Government officials who don’t like a law play to lose.
Nobody is looking out for drivers.
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