The IIHS got some press this week when they found reducing Boston’s speed limit did not slow down Boston’s traffic. That’s what the data said. They lied to the press, saying the city cut down the number of speeders, and got their spin all over the news.
Anybody who knows how speed limits work knows reducing the unposted speed limit from 30 to 25 is not going to slow traffic. The IIHS data proves it did not. Here is that data:
|Period||Mean||85th||Over 25||Over 30|
Before and after speeds were exactly the same. The average speed was 24.8. The 85th percentile speed was 31.0, meaning 15% of drivers were going over 31.
They also measured speeds in Providence, where the speed limit did not change. As in Boston there was a change of about 1% in the fraction of vehicles exceeding 25, 30, and 35 mph thresholds. If Providence’s speed distribution figures changed by a percent or so without a speed limit change, a similar change in Boston could not be attributed to the speed limit change. It’s just what you expect from random chance.
It’s well known in the scientific community that if you know your conclusion ahead of time, you can dissect the data to “prove” your point. When traffic speeds came back the same before and after the IIHS ran models until they found one which said a little corner of the data fit their agenda. That’s PR, not science.
This is not the first, and probably not the hundredth, publication to find changing speed limits does not change how fast Americans drive. The best known study is the Parker report. The federal DOT hired a consultant to prove that low speed limits slowed traffic and improved safety. Defying his sponsors, he found low speed limits had no effect on traffic speed. His report, which the National Highway Traffic Safety Administration tried to suppress as politically unsound, helped persaude Congress to repeal the national speed limit in 1995.
The suggestion that pedestrian deaths might be reduced by the speed limit change is also unsupported by evidence. When Boston was about to reduce the speed limit I looked at the last available year of crash reports (2013 at the time) and found the corresponding local news stories about fatal pedestrian accidents. Four were low speed accidents in busy areas where somebody wasn’t paying attention. One was caused by a drunk driver who went onto the sidewalk. The last was caused by a man who ran into traffic on a state road not subject to the city speed limit. Reducing Boston’s speed limit would not have prevented any of these.
You’re much more likely to die under the wheels of a low speed bus, truck, or duck boat than you are to get hit by a driver going 40 mph on a city street. That’s why figures like X% fatality rate for pedestrians hit at Y mph don’t matter. A weight limit would make a difference. A speed limit does not.
The IIHS is an arm of the insurance industry. One revenue source for the insurance industry is surcharges for speeding tickets. They call themselves the Insurance Institute for Highway Safety. People who have seen them in action over the years say the Insurance Institute for Higher Surcharges. They want more speeding tickets.
Take a second look at the chart above. The figure 18.2% is the fraction of speeders before the speed limit change. The figure 46.9% is the fraction of speeders after.
Here’s a more honest headline: Speeding triples in Boston after speed limit reduction and the ticket industry couldn’t be happier.
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