To support the latest plea for a speed limit reduction, Somerville, Massachusetts politicians resurrected a ludicrous claim, a “health impact assessment” predicting that reducing the unposted speed limit from 30 to 25 will cause a huge reduction in death.
Here’s where it goes off the rails: “Though this bill reduces the default speed limit on local roads by 5 mph, actual speeds are only expected to drop by 1.8 mph.”
Traffic speeds will not change at all. Nobody with experience in American traffic policy would expect them to.
The authors were two grad students and an undergraduate studying epidemiology, environmental studies, and social and behavioral science. They’re out of their field. If you asked me to model the effect of temperature on arbovirus transmission I might get it wrong. (That’s actually a bigger public health concern than urban speed limits, and one I leave to the experts.)
So where did this claim come from? They found a graph in a review article titled “Speed Limits, Enforcement, and Health Consequences.” The review is mostly self-citing by the author. Literally, most of the references are to the author’s previous publications.
The graph plots a curve through noisy data relating speed limit changes to travel speed changes.
So where did the graph come from? The paper is supposed to be a review, not original research. There is a citation to one the author’s earlier papers, but the graph isn’t there. (I did find a chart on page 54 saying the speed limit has little effect on urban road safety.)
I don’t know exactly where the data came from, but I know where it didn’t come from. If those data points are valid at all, they mainly refer to experience in countries where speed cameras rule.
We know the graph is wrong in America because we’ve done the experiment here.
The best known of many studies is probably the Parker report from the 1990s, which showed the effects of speed limits on travel speed “are not sufficiently large to be of practical significance.” It was responsible for repeal of the national speed limit in 1995. Even NHTSA, which hates its conclusions, has been forced to acknowledge that it exists.
Why isn’t this well-known study cited? The funding agency wanted a press release to support a speed limit reduction. They want to believe they can flip a switch and control a million drivers. That’s an impossible dream with today’s technology.
I could go on and explain why the analysis of the effects of a 1.8 mph reduction in speed are wrong, but if there is no reduction then obviously there is no effect.
Here’s one clue. If you read past the introduction you’ll see a consultant told them most drivers around Boston are stuck in traffic wishing they could drive the speed limit.
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