Think speed limits are set for safer driving? Find out what they're really about.

I'm not a particularly fast driver (well, not unless I'm listening to NPR's "Fresh Air" with Terry Gross), but even I wonder whether our current limits do more harm than good. I love it when videos like this shatter my intuitions about things I take as normal.

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Speaker: In past episodes, I have talked about how accidents are the number one killer of children, and how car accidents represent a fairly large part of accidents in general. So why don't we just lower the speed limits? I'm glad you asked. That's the topic of this week's Health Care Triage.

There seems to be a widespread belief that faster driving is unsafe, and slower driving is much better. Certainly, the fact that we have speed limits reinforces this perceptions, but were national speed limits set for safety? The national speed limit of 55 miles per hour was created by the Emergency Highway Energy Conservation Act of 1974.

As the name implies, the law wasn't a response to safety concerns. It was a response to the OPEC oil embargo of the 1970s. The hope was that driving slower would cause Americans to consume less gasoline, and be less dependent on foreign oil. Many people and states didn't like this, but congress basically tied federal highway money to compliance with the law. Even then, a number of states pushed the limits. In the end, though, it was estimated that the law reduced consumption by about 1%. In 1987, congress relaxed some of the limits, and let the speed limit go to 65 miles per hour on certain interstate highways. The law was fully repealed in 1995, and speed limits reverted to state control. Speed limits went up in many states, and concerns arose about an increase in accidents and fatalities.

After the repeal, the number of fatalities went up by 90 in 1996, and by 60 more in 1997. This wasn't too much of a surprise for those of us who'd read a 1992 report by the Federal Highway Administration, which studied twenty two states, over five years, and found that raising the speed limit didn't increase accidents or even really speeds. People don't really follow speed limits as much as we'd like, anyway. And back in 1987, when they relaxed the maximum for the first time, fatalities went down.

But what we really care about isn't the number of deaths, but the rate of deaths. After all, if more people are driving, or if they're driving for more miles, we should expect there to be more accidents. In 1995, before the national speed limit was repealed, the fatality rate was 1.7 per a hundred million miles traveled. In 1997, two years later, it was 1.6. There were also lower rates of injuries in 1997 than 1995. the injury rate before the repeal of the speed limit was 143 per hundred million miles traveled. Two years after repeal, it was down to 133. Yes, there are studies that point to the fact that fatalities are higher in states that have raised their speed limits than in states that haven't, but fatality rates were higher in those states even before they repealed the law. Moreover, the fatality rates in the states that have repealed the law have still dropped since repeal.

Now, I'm not advocating that we don't need regulation on the highway. We don't want people driving unsafely. But there's a decent amount of evidence that shows it's not driving fast that's the real danger, it's driving at very different speeds. In a seminal paper published in The American Economic Review in 1985, Charles Lave showed that there's real association between the fatality rate and the average speed of drivers. As long as the cars are all traveling at the same relative speed, fast or slow, the fatality rate is low. When speed limits are set artificially low, then more variation occurs. This can lead to more accidents. It can lead to more deaths.

Which leads me to one of the most frustrating bits of policy research asynchrony I've seen. It involves Route 3, in Massachusetts. The speed limit for Route 3 used to be 60 miles an hour, and it had been for decades before the national law reduced it to 55 in 1973. When the law was repealed, the State Highway Commissioner decided to keep the limit at 55, though, because it was thought that if the rate was raised and then an accident happened, the state might be sued. More than a decade ago, Route 3 was rebuilt, and when it was, it was designed for a speed of 68 miles per hour. That means it's totally safe to drive it at that speed, even in less than desirable conditions. In 2006, Massachusetts traffic engineers recommended an increase in the speed limit. Evidently, state police fought it, since they can make a lot of money writing tickets for those who ignore the limit. They won.

But in 2008, a report was prepared for the Massachusetts highway system. Why? Turns out that the fatality rate on Route 3 is way higher than the national average. They came up with a number of suggestions, including this, and I'm quoting:

"A more substantive change is to possibly change the legal posted speed from 55 miles per hour to 65 miles per hour. The thesis is that with speed limits raised, the speed differential will be effectively reduced. Discussion by the RSA team noted that the large speed range could in fact be influencing the frequency of lane change maneuvers that currently occur, increasing the risk of an incident. The more lane changes that occur increase the possibility of driver error or errant vehicles."

I know it makes intuitive sense to many of you. Driving slower is somehow safer. I wish it were that simple. But, as with many things in life, what seems intuitive sometimes isn't the case. We have to be willing to let go of our biases. Sometimes, not always, faster is safer.

There may be small errors in this transcript.

This bit was put together by the folks at Healthcare Triage. The gentleman s Dr. Aaron Carroll. He and his team are total champs because they also put together two blog posts laying out the evidence, reports, and sources for this talk, as well as answers to viewer questions.

Jun 16, 2014

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