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August 05, 2004

Speed Limits and Safety

I've long been dubious of the 55 mph speed limit. It might save lives, but at a huge cost which someone ought to calculate. BUt I just had a thought that makes me doubt whether it even saves lives.

Consider this. Suppose you are trying to avoid a thunderstorm and you are driving 100 miles. If you drive at 50 mph, you have double the chance of being in the storm that you would if you drove 100 mph. Driving faster is less safe.

Suppose, now, that the main danger on a road is from a drunk driver. The more time you spend on the road, the more time you might run across him and be hit. In this case, too, driving faster is driving safer.

The question, then, is whether the extra danger of more hours on the road from a slower speed is offset enough by the extra safety of being better able to avoid hazards. I don't know the answer.

One background fact that would be useful is how many and how major are accidents in town, at slower speeds, compared to on highways, at higher speeds. I should look that up if I can.

Posted by erasmuse at August 5, 2004 08:14 PM

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As I remember Nixon put in the double-nickle (55) speed limit to save fuel. Subsequently there was a big hubbub in the media that the death rate from auto accidents went down. Of course they neglected to mention that car use had gone down in that period due to high fuel prices and a poor economy.
Personally I don't see much kinetic energy decrease from 65mph to 55mph or with a slightly prolonged expansion of time to avert a problem. I also have difficulty with this factoid --- most accidents occur within 1.5 miles from home and at speeds of 30 mph or less.

A great many more lives could be saved by a speed limit of 30 mph on the highways, but most individuals would commit suicide rather than travel at such a speed.

Correct me if I am wrong but was not the interstate highway system designed to be safely traveled at 65mph.

I notice that all big trucks seem to have an air flow modifier on them to reduce air resistance.

I think that the savings in fuel costs would be wiped out by the increased driver behind the wheel costs.

Posted by: Jim at August 6, 2004 02:43 PM

Actually, I wonder whether more lives *would* be saved by a limit of 30 mph on highways. That would mean twice the density of cars. Drunks and teenage dragsters might still go 80 mph. True, each accident would have less kinetic energy. But if there were enough more accidents, there might still be more fatalities.

Posted by: Eric Rasmusen at August 8, 2004 11:21 PM

Surely if the vast, law-abiding majority had to travel at 30 mph the much gentler impact of car against other car or against tree, etc. plus the slower eye-muscle reaction time needed to avoid collision would result in less fatalities.
But, of course, 30 mph is a completely unrealistic possibility.

Posted by: m. rasmusen at August 9, 2004 10:08 PM

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