Are Speed Limits an Exception?

Priceonomics asks, “Is Every Speed Limit Too Low?”  Seriously, read the entire article, but here are some teasers:

The answer lies in realizing that the speed limit really is just a number on a sign, and it has very little influence on how fast people drive.


The speed drivers choose is not based on laws or street signs, but the weather, number of intersections, presence of pedestrians and curves, and all the other information that factors into the principle, as Lt. Megge puts it, that “no one I know who gets into their car wants to crash.”


This is important because, as noted in a U.S. Department of Transportation report, “the potential for being involved in an accident is highest when traveling at speed much lower or much higher than the majority of motorists.” If every car sets its cruise control at the same speed, the odds of a fender bender happening is low. But when some cars drive 55 mph and others drive 85 mph, the odds of cars colliding increases dramatically. This is why getting slow drivers to stick to the right lane is so important to roadway safety; we generally focus on joyriders’ ability to cause accidents — and rightly so — but a car driving under the speed limit in the left (passing) lane of a highway is almost as dangerous.


The choice of the 85th percentile speed is a data-driven conclusion — as noted Lt. Megge and speed limit resources like the Michigan State Police’s guide — that has been established by the consistent findings of years of traffic studies.


Yet most speed limits are set below the 85th percentile speed.


It seems absurd that over half of drivers technically break the law at all times. It’s also perplexing that speed limit policy so consistently ignore traffic engineering 101.


The other reason speed limits may remain low, which John Bowman, Communications Director of the National Motorists Association strongly insists on, is that cities and police departments use traffic citations as a revenue generating tool.


So, all this is important in its own right, and we should think carefully about speed limits, especially in the potentially coming days of driverless cars.  But more, I’d like to encourage my readers to think broadly about what this implies for other laws.

Speed limits are well-studied because they directly affect nearly everyone in the nation, because it happens that it’s relatively easy to quantify their effects, and because it happens that traffic accidents are one of the biggest causes of preventable death in the United States.  And, at risk of belaboring the point, what they find is that:

  • Functional policy is at odds with what the data suggest is the best policy.
  • The citizenry is not well-informed about what the best policy is.
  • The government seeks profit, and in this case does so by undermining public safety.
  • People routinely ignore the laws.

Now, ask yourself — do you think that other common laws are at odds with the best policy?  Do you think that the democratic watchdogs of those laws are well-informed about them?  Do you think that nobody is profiting from the existing regulation?  Do you think that those laws are well-enforced?

In other words, are speed limits the exception or the rule?

If you think speed limits are the exception, why?  What is it that’s unique about speed limits that draws this kind of bad policy?

I would argue that speed limits are probably much better policy than most laws.  They actually have a unique amount of data  collected about them, and so there are these people and organizations seeking to rationalize them.  Most — probably 95-99% — of the laws out there are much less well-studied.  Why would we imagine that they are better than speed limit policies?

This is the kind of thing which informs my low prior probabilities that any given local regulation is well-founded.

As a bonus question for you, if we do think that there are a relatively large number of bad laws out there, how do we imagine they can be corrected?  Is it genuinely possible for the democratic process to work individually on each one?  If it is, why haven’t speed limits — again, a uniquely out-in-the-open case — been reformed already?


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