I think there are a lot of contributing factors to road rage, and the road infrastructure is one of them. Artificially low speed limits, poorly timed traffic lights, and poorly designed interchanges all add to driver frustration. I was reading in the National Motorists Association blog today that Michigan authorities have started to address this problem by setting reasonable speed limits on some of their thoroughfares.

On Interstate 496 outside of Lansing, MI, the speed limit had been 55mph. Some drivers drove the speed limit (and probably some in the left lane). Some drove above the speed limit at speeds that were probably safe, despite being illegal. This differential resulted in people jumping around between lanes, tailgating, and exhibiting other hallmarks of aggressive driving. When authorities raised the speed limit to 70mph, all these behaviors stopped because everybody was going roughly the same speed (and the number of accidents decreased too).

Contrary to what the fear-mongers will tell you, increasing speed limits has negligible impact on average speeds. It does, however, have a positive impact on flow. Most traffic engineers agree that speed limits should be set at the 85th percentile of free-flowing traffic in good conditions (i.e., if 85% of cars on a road on a clear day are going 65mph or under, the speed limit should be set at 65mph). Most states, however, set their speed limits much lower than this so they can raise more revenue from punitive speeding tickets.

Speed limits should be set for safety reasons, not to generate revenue…and the evidence suggests that higher average speed limits are, in most places, safer.