By Ben Langlotz
A. 55 in a 55, when the flow is 65. The limit is unreasonably low, so "the flow" is much faster, but "A" wants to avoid a stressful encounter with the police, a ticket, and an insurance hike. Other "A"s don't wish to show disrespect for society's rules.
B. 80 in a 65, when the flow is 70. The flow is slowed by a low limit and fear of tickets, etc. Enforcement is rare, so the real risk of a ticket is low due to safety in numbers. A hurried, attentive motorist "B" is going to be over the limit anyway, so why not speed by enough to make a real difference in arrival time, B reasons.
C. Frightened 45 mph elderly and decrepit Beetles.
D. Racers and suicidal maniacs.
A and B are the most common reasons, C and D are "too" common, but easily less than 1% of motorists, so have an insignificant effect on the number of accidents caused by overtaking/speed differential.
(Note: The average driver will overestimate C and D, since he won't see the bulk of cars that are around the same speed but never pass by, but will notice C (passed by all) and D (passing all.) C and D ARE safety hazards, but better addressed by measures other than low speed limits.)
Suppose we raise limits to 65 urban/ 75 rural interstate. Motorist A is now happy to go the limit, along with the bulk of all motorists (85% at or below the limit.) History shows that we may see about a 2 mph increase in the average speed, but much of this must be coming from A and his bretheren. (And the few "A"s that can't handle the limit will keep right, since they no longer have the righteous sense of being a "citizen safety enforcer.") But will the previous flow go faster? Consider the case of B.
B drove 80 in a 65, when 2/3 motorists were violators. Now (75 mph limit), he can avoid the paranoia and distraction of watching out for officers by keeping it a bit under 80 - about his old speed. Most of the B types will be happy to "get there" in the same time as before. Those few who think they can still do 10 or 15 over, like before, will no longer find friendly convoys of violators. They will see few officers already occupied with a 74 mph "perp," as before. Now, the officer will be either enforcing lane discipline, observing impaired or inattentive drivers, or hiding in a speed trap waiting for the true sprinters. It won't take long for even most of the remaining speeders to realize that they aren't getting "lucky" about tickets anymore. Increased complieance by the bulk of motorists effectively exposes and makes noncompliance stick out like a sore thumb.most drivers.
But what about C and D? C (our kind but frightened elderly driver) will no longer find that 1/3 motorists are driving at 55. Now, his 45 or 50 mph speed enjoys little company. Virtually EVERY car is now flying by at 15-20 mph faster. Dangerous? Yes. But C will undoubtedly perceive the danger as never before, and may reduce or eliminate freeway driving. Put another way, the low limits that cause 1/3 of drivers to go 55 are "enabling" those least capable drivers to remain on high speed highways where they don't belong.
And D? Like the last few stubborn Bs above, he will stick out like a sore thumb. Remember that the Federal Highway Administration study of the effects of raising and lowering limits found on interstates raised to 65, that while speeds in most percentile ranges increased by 0-3 mph, the 99th percentile speed DROPPED. My model may seem optimistic, but it fits with the facts.
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