Tuesday, November 24, 2015

Speed limit fallacy


It seems reasonable to assume that driving slower is safer.  After all if you are not moving, you can’t crash.  You will never get anywhere either, and you can still be crashed into.  While it is true that driving slower reduces severity of accidents there is no evidence that it reliably prevents them.  Most fatal accidents happen at below 35mph!  This is simply because most driving time is spent below 35mph, in locations where collisions are likely to occur, like urban intersections.  On the other hand when all vehicles are going in the same direction at the same speed, as in a tunnel, they cannot collide.  If they do get a little out of sync and collide the severity is reduced.  This is the rationale to build limited access divided highways. The safest situation on traditional roads is to discipline traffic so that the difference in speed among vehicles is minimized.  This can be done two ways.  Draconian enforcement or rational speed limits.  Unfortunately there are not enough police or courts to make the former viable, unless we want to give the police shoot on sight authority, but that has problems of its own.   

It is widely believed that no matter what speed limit is posted most people will cheat by 5 to 10 miles per hour.  Many people also think the police will give them 5 mph or 10% grace. Neither is true.  It has been scientifically established that if there is no posted limit on a highway 85% of the drivers will drive at a safe and reasonable speed for the conditions.  The traffic will thus be self disciplined and inherently safer.  More than half will be within a 10 mph range of speed, with many going slower for personal reasons, and a few going a little faster.  Of course there will be a small number going outrageously faster.  These are the ones enforcement should be concentrated on.  Unfortunately enforcement creates revenue and that can become the motivation for increased enforcement activity.

Untrained politicians and bureaucrats believe in the fallacy, and given the chance almost always decide to post the speed limit 5 to 10mph less than good traffic engineering dictates, 10mph slower that they themselves do drive on the same roadway.  When their prophesy comes true, their solution is to lower the limit by another 5mph.  This has almost no discernible effect on the maximum speed. Instead the traffic gets more chaotic and dangerous, because while small percentage will rigorously obey, the crazies (habitual speeders) will try to drive as fast as ever.  The rigorously obedient will frustrate not only the crazies, but many otherwise safe drivers, who will now tailgate (follow unsafely close) and be tempted to pass unsafely.  A driver who might feel safe driving 55 on a certain highway will probably not take many chances to pass one driving 50, but as the speed of the impediment decreases the willingness to pass increases.  As the motivation to pass increases the conditions under which a driver will attempt to pass deteriorate.  Almost no one would hesitate to pass a farm tractor going 7mph.

Another problem is boredom.  Bored drivers minds may wander or they may get drowsy.  I don't think anyone in a race ever fell asleep at the wheel, but it is  a real hazard on long boring tips at speeds that require more attention to the speedometer than the road.

On a highway the grouping of traffic can be observed.  The slower a vehicle is travelling the closer the following traffic will be, disproportionately closer.  Likewise the frustration and risk tolerance of the following drivers will increase.  On a narrow highway for example a car travelling at 35 will accumulate a large number of cars behind it, with at least one obviously tailgating.  A car travelling at 45 will have fewer cars following, with hardly anyone tailgating.  A driver at 55 will probably have none behind him, until he overtakes one of the slower drivers above.  Too-low speed limits decrease safety.


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