Bryan Caplan  

Public Opinion as a Political Speed Limit

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Suppose someone said: "People drive 5-15 miles over the speed limit. It's obvious, then, that speed limits have no effect on how people drive."

It's a pretty silly argument, isn't it? People drive 5-15 miles over the speed limit in large part because they can get away with it. If they started going much faster, though, they would rack up a pile of speeding tickets. And if you raised the speed limit by 10 miles, they'd probably start driving about 10 mph faster.

OK, suppose someone said: "Politicians moderately deviate from what the public wants. It's obvious, then, that public opinion has no effect on policy." This is one of the major objections to my book expressed by Daniel Casse in the Wall St. Journal, and most of the panel at Yale Law School.

Again, I'd say this is a pretty silly argument. Politicians deviate moderately from public opinion is that they can get away with it. If they started deviating much more, though, they would be putting their jobs at risk. And if public opinion changed, politicians would probably change their position by about the same amount.

In both cases, the logic is the same. You can usually bend the rules and get away with it, so most people bend the rules. That does not imply, though, that the rules don't matter.


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COMMENTS (9 to date)
Vincent Clement writes:

People speed not because they can get away with it. They speed because they are driving at or near the design speed limit, which more often than not, is much higher than the posted speed limit.

So, yes, speed limits have no effect on the way people drive. They drive according to road design and conditions and traffic flow. You could eliminate speed limit signs on most interstates and most people will drive between 65 to 75 mph. I recall that when Montana had no speed limit during daylight hours, the average speed of vehicles dropped.

Butter writes:

I think his question to you is, "Why then isn't economic policy much worse than it is?" And I think that is a legitimate concern with your theory. Witness Obama and Hillary. Both are spouting protectionist rhetoric to garner popular support, but no one expects either of them to actually pursue protection once elected (Obama even reportedly called the Canadian Ambassador and assured him that this was all for show and NAFTA was safe).

mgroves writes:

I would also contend that where a speed limit is known to be strictly enforced, people are much less likely to flirt with going a little faster (i.e. speed traps).

So, maybe there should be some overcompesation to take this into account? Maybe like, I don't know, some sort of federalist government that limits the effect that laws can have and brings the consequences as close to the people it affects as possible?

GuyF writes:
I recall that when Montana had no speed limit during daylight hours, the average speed of vehicles dropped.
That just refutes what you said three sentences ago, that the speed limit has no effect on how people drive.
Vincent Clement writes:

GuyF: I apologize, I should have expanded my comment.

When I said "speed limits have no effect on the way people drive" I meant POSTED speed limits have no effect on the way people drive.

And I'll go one step further. POSTED speed limits that are NOT the same as or close to the DESIGN speed limit will have no effect on the way people drive.

The Montana example shows that people were already driving at or near the design speed limit with or without a posted daytime speed limit.

John Fast writes:
So, yes, speed limits have no effect on the way people drive. [...] I recall that when Montana had no speed limit during daylight hours, the average speed of vehicles dropped.

Let me make sure I understand your post: Speed limits have no effect on the speed people drive, and removing them changes the speed that people drive.

Am I overlooking something here?

John Thacker writes:

Let me make sure I understand your post: Speed limits have no effect on the speed people drive, and removing them changes the speed that people drive.

Am I overlooking something here?

As a probabilist, I have no problem with someone saying that "there is no effect," and then noting that in one particular case the effect was a small opposite of the hypothesized result. That's exactly the sort of thing that would disprove the hypothesis that people always drive to 5-15 over the speed limit.

Speed limits have little significant effect on the speed people drives. There is a slight effect, but it's nowhere near the claim that people drive 10 mph faster if the speed limit were raised by 10 mph, mutatis mutandis. In fact, in one particular case in Montana, the average speed actually dropped slightly when eliminating them. This is such a counterintuitive result that it stands to reason that it is largely random noise, and indicative of how little significant effect there is.

I have driven on the interstate in a variety of East Coast states, and find little difference in average speed of traffic regardless of speed limit. (It depends a lot on traffic and road conditions, though.) The chance of being pulled over is simply too low, especially when the entire flow of traffic is proceeding at the same speed. The same is true of local roads; I've seen quite a few that have had speed limit changes (up or down) with almost no effect whatsoever in average speed.

John Thacker writes:

From a US Department of Transportation and Federal Highway Administration study on the effects of raising and lowering speed limits:

The results of the study indicated that lowering posted speed limits by as much as 20 mi/h (32 km/h), or raising speed limits by as much as 15 mi/h (24 km/h) had little effect on motorist' speed. The majority of motorist did not drive 5 mi/h (8 km/h) above the posted speed limits when speed limits were raised, nor did they reduce their speed by 5 or 10 mi/h (8 or 16 km/h) when speed limits are lowered. Data collected at the study sites indicated that the majority of speed limits are posed below the average speed of traffic. Lowering speed limits below the 50th percentile does not reduce accidents, but does significantly increase driver violations of the speed limit. Conversely, raising the posted speed limits did not increase speeds or accidents
John Thacker writes:

Although that site ends up being a little conspiracy sounding for my tastes, all research that I've seen comes to similar results. The effects of changes in speed limits on average speeds are small. Here's another study, hosted on the Ohio DOT site.

Speeds don't change much with speed limits. A 10 mph change in speed limits tends to have around a 1 to 2, if that, change in average speed.

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