Fact-Checking the Toyota Hearing: Lower Speeds Increase Safety

Megan McArdle at the Atlantic, writing on today’s Toyota hearing in the House oversight committee, hears Transportation Secretary Ray LaHood claim that "lowering the speed limit to 30 mph would not save any lives, which is why we have minimum speeds on highways."

lahood.jpgLaHood, at left, with the president at right. (Photo: whitehouse via Flickr)

Leaving aside the gaping logical hole in that statement — which Robert Mackey of the New York Times suggests (check out the 12:04 post here) may have come from Souder’s argument that lower speed limits would save more lives than "100% safe" cars — there is plenty of research out there pointing to the beneficial effects of lower speeds on safety.

Traffic author Tom Vanderbilt recently cited the impact of 20 mile-per-hour urban speed zones on reducing road injuries in the United Kingdom, and a 2007 study by the AAA Foundation for Traffic Safety [PDF] outlined the following "general rule of thumb":

When travel speed increases by 1%, the injury crash rate increases by about 2%, the serious injury crash rate increases by about 3%, and the fatal crash rate increases by about 4%. The same relation holds in reverse: a 1% decrease in travel speed reduces injury crashes by about 2%, serious injury crashes by about 3%, and fatal crashes by about 4%.

Could LaHood be unaware of the relationship between lower speeds and decreased risk of injury? It’s certainly possible — despite the former GOP lawmaker’s good record on infrastructure reform and sustainability, both in concept and in practice, he remains a relative newcomer to the nitty-gritty of transportation, as the Times reminded readers in a highly readable profile last year.

  • Engineer

    Ignoring the underlying issue that 30 MPH speeds on an interstate highway would suck, I agree that lower speeds would certainly reduce injuries. However, lower speed limits are an entirely different question. Speed limits only work when they’re largely self-enforcing. Good luck getting nearly all drivers to do 30 MPH on a road designed for 70 MPH. What you’ll actually do by lowering the speed limit is increase the speed variance, which increases the crash rate.

  • J:Lai

    Lower speed limits (with enforcement) in urban areas would improve safety.
    Lower speed limits on grade-separated highways don’t seem to offer many benefits. I don’t see anything wrong with drivers on interstate highways traveling 70 mph.

  • Yeah the comment seams aimed at highways. There is a very small difference in safety from 30 to 50 to even 70mph.

  • Lower speeds increase safety, because the more responsible drivers are likely to drive more slowly. But lower speed limits don’t increase safety, because the irresponsible drivers just drive more irresponsibly in other aspects. This is Smeed’s Law, which states that car accident fatality rates depend only on the number of cars on the road.

  • BicyclesOnly

    A lot of the speeding going on is not individuals who don’t care what the limt is. Probably the largest group of speeders are the substantial minority of drivers, perhaps the majority, who follow the “rule” than speeding up to 9 MPH over the limit is OK. There is another, bolder set of speeders who will exceed the limit by more than 9 MPH when part of a pack of drivers who are doing so (which in my experience as a driver only occasionally happens at speeds of 20 or more MPH over the limit). The actual limit is an important reference point that keeps most speeders near, if not under the limit.

    An excellent reason to reduce NYC limits to 20 MPH!

  • car speed is always effecting accident ratio but there are lot of other things in the manufacturing of the car which effect this ratio


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