Anti-car activists in California released a report targeting the speed trap law. The speed trap law is much hated because it is effective. In other states city councilors can hand out signs and speed traps like candy to favored constituents. In California the law says speed limits on major urban streets can’t be below the average speed of traffic.
The name of the committee gives you a clue what the report is going to say. It is called the “Zero Fatalities Task Force.” You can expect them to be slogan-driven instead of fact-driven. The proof came on the first page I read, the executive summary of an alleged research synthesis.
Here is the opening of a key paragraph:
Drivers have a tendency to underestimate speed. This can range from an underestimate of 10% at higher speeds (70 mph) and up to 30% at lower speeds (35 mph). This demonstrates that drivers have limited capability to self-regulate a safe speed, especially at lower speed areas. It is therefore undesirable to rely on operating speed to
establish safe speed.
This is not just mistaken, it reflects a level of ignorance that can only be achieved by ignoring the real literature instead of synthesizing it.
The 85th percentile rule for speed zoning was based in part on an observation of driver behavior. Drivers going near the average speed of traffic, especially slightly faster than the average speed, had fewer accidents. It was not based on any consideration of how many miles per hour drivers were going or thought they were going, either in absolute terms or relative to signs or statutes. When the rule was developed many roads didn’t have speed limits. It didn’t matter if drivers thought they were going 40, speedometers said they were going 50, and a stopwatch proved their true speed was 45.
This is the same mistake that earned Australian researchers a lot of press in 2016. Somebody who saw a picture of a road might say it was safe at 50 km/h while somebody who drove on the road would choose 60. The difference is not dangerous. It may point to a problem in estimating kilometers per hour. It may point to the desire of experimental subjects to try to give the “right” answer. It may point to failure of a picture to tell a story. But it is consistent with a principle I keep repeating: the average person behind a wheel is better than the average person behind a desk when it comes to choosing a safe speed.
When approaching a curve we do not translate miles per hour into feet per second, estimate the curve radius in feet, and solve a quadractic equation in our head to determine whether we need to brake or whether the available lateral acceleration is sufficient. For every math graduate student in the NFL there are a thousand successful professional athletes who couldn’t pass a high school physics test. There is some evidence that being smarter than average doesn’t make you a safer than average driver. Driving is instinctive, not intellectual.
Estimating miles per hour tells us nothing about ability to “self-regulate a safe speed.” The real lesson is the opposite of the one offered. If drivers really can’t keep below a mile per hour target then speed limits are doomed to failure as speed management devices. If you post the actual target speed, traffic will not hit the target. If you post the speed limit 30% below the target speed, every driver on the road gets a ticket. Which seems to be what the Vision Zero folks want. But it’s not what anybody outside that lunatic fringe should want.
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