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TinotopiaLog → Speed Bumps (18 Jul 2001)
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Wednesday 18 July 2001

Speed Bumps

I live in Northern Virginia, which has got to be the speed bump capitol of the world.  Or maybe speed bumps have just become very popular since I’ve moved here.

In any case, there are speed bumps everywhere.  There’s one outside the local IKEA store that causes normal passenger cars to bottom their suspensions and scrape the ground (artificially high because of the height of the speed bump) when the cars are loaded with furniture.  There are dozens outside the local grocery stores, no doubt financed in part by the egg industry.  There are some on normal through streets, on the way to loading docks, in parking garages, etc., etc., etc.  In short, there are a lot of them.

And they typically don’t do their job.  Presumably, the point is to keep the cars from going faster than the local authorities would like.  (Never mind that there are actually good ways to do this: Discussion of that is beyond the scope of this document.)  What they do is reduce the average speed of the cars.  The problem is that they do this by forcing the drivers to almost come to a stop before each bump.  Drivers don’t like this, so they speed up out of frustration once they’ve lurched their car over the thing.  I don’t think that this is the intended result.

This problem was solved, though, in 1953 by Nobel-prize-winning physicist Arthur Holly Compton (see photo at left).  Compton won the Nobel in 1927 for the discovery of the Compton Effect, the increase in the wavelengths of X rays and gamma rays when they collide with and are scattered from loosely bound electrons in matter.  During World War II, he was director of the Metallurgical Laboratory at the University of Chicago, where he was instrumental in the establishment of the first controlled uranium fission reactor — he was Fermi and Szilard’s boss, basically.  

After the war, in 1953, Compton was chancellor of Washington University in St. Louis.  Apparently the job didn’t require all of his attention (he had won a Nobel prize, after all), because he spent at least some of his time looking out the window of his office in Brookings Hall.

He noticed that cars tended to speed along the road in front of Brookings Hall, and he designed some speed bumps to discourage this.  His speed bump (bumps of this design are now usually called "speed humps") was a double-ramp kind of affair, as in his drawing below:

His calculations indicated that the bump would cause differing amounts of upward acceleration on the car at different speeds:

5 mph
10 mph
20 mph
30 mph
40 mph
50 mph

That is, the car becomes airborne at 30 mph.  At 20 mph, though, the ride is fine, and the bump is hardly noticeable at 10 mph.

I can personally verify both of those assertions.  A friend of mine once drove a Chrysler K-car at one of these things at 50 mph.  (He was a student at another school, and did not have the right amount of respect for Compton, or, for that matter, physics.)  The result was a lot of sparks, a missing exhaust system, and a dented roof (dented from the inside, by his head).  The trunk also popped open, presumably from a distortion of the entire body of the car upon landing.  These things are actually installed in pairs, which makes them much, much nastier.  If you drive the recommended speed, though, they don’t present a problem to any car I’ve ever driven.

Most speed bumps are particularly harsh on sports cars.  They tend to have stiffer suspensions and less ground clearance.  I once drove a car that had about 3.5" of clearance: a lot of speed bumps are higher than that, so I just had to avoid them for fear of winding up high-centered.  The 3.5" clearance car had no problem whatsoever with these, though.

These bumps are more expensive than the lump-of-asphalt variety, though, so they’re not used in many places.  This would probably change if drivers whose cars were damaged by the inferior kind would diligently submit claims for damages.

A.H. Compton Speed Bumps (at Washington University)

Posted by tino at 13:00 18.07.01
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