The automatic excuse generator

One of the important jobs of a government traffic engineer is coming up with excuses for low speed limits. I call this the “public relations engineer” role.

Now the Federal Highway Administration has an automatic excuse generator.

I saw it in action when I read a story about a speed limit change in rural New Hampshire. The lead was “speed limit raised” but should have been “speed limit set 10 mph too low.” The excuse was a “web-based tool” recommended 40 mph.

The tool is called USLIMITS2. It’s a simple computer program that collects excuses government officials use to reduce speed limits.

For 70 years the recognized method of setting speed limits has been to post the speed limit 5 to 10 miles per hour above the average speed of traffic. This is the “85th percentile rule.” It was motivated by safety research and there are independent policy considerations to use it.

USLIMITS2 pretends to respect the rule, but let’s try it out…

Say you have an interstate highway in the middle of nowhere. Plug in the traffic speed (80), accident rate (none), and development (none), and it will spit out an answer: 75. What happened to the 85th percentile rule? One of the excuses is speed limits should never be over 75 on Interstates and 65 on two lane roads. Because that’s what the group they surveyed was comfortable with.

It goes on and on for pages of flowcharts, and none of them is justified by safety research. It’s all based on asking people “what would you do?” They asked police officers, judges, and politicians. I wouldn’t post 80 anywhere, I wouldn’t use the 85th percentile rule in a built-up area, etc. And they tried to please them all.

The most important factor in the report was a supposed increase in traffic fatilities when speed limits were raised in 1986 and 1995, based on a disputed retrospective analysis. The scientific method says design the experiment then collect data; if you design the experiment to fit the data you can prove anything. Somebody did a properly controlled study (the Parker report). It was mentioned and forgotten. The people surveyed didn’t want to believe that reducing speed limits wouldn’t help.

So USLIMITS2 is designed to set speed limits around the median (50th percentile) speed, or 5 to 10 mph too low.

The sad thing about the current state of affairs is, if followed this would be an improvement. Every road I drive regularly that has a posted limit, has a posted limit below the average speed of traffic.

That’s what happened in New Hampshire. A speed limit that probably should have been 50, possibly 45, was set at 40 instead. That was an increase.

The good news is local politicians smelled a rat. They understood that making verybody into a speeder didn’t serve a legitimate purpose. This happened near Keene, the unofficial capital of the libertarian Free State project. Let’s hope this skepticism spreads closer to big cities.

The opinions expressed in this post belong to the author and do not necessarily represent those of the National Motorists Association or the NMA Foundation. This content is for informational purposes and is not intended as legal advice. No representations are made regarding the accuracy of this post or the included links.

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