By John Carr, NMA Massachusetts Activist
Last time I suggested we were barking up the wrong tree by quoting statistics for car-pedestrian crash survivability.
A 1995 article in the British Medical Journal asked how often people were hurt by cars. “How dangerous is the street?” is a more interesting question than “how dangerous is the crash?”
The study measured road and traffic conditions at sites where children were struck and injured by motor vehicles. They also looked for controls, children who were not hit by cars.
The study says it’s bad to be poor, dark-skinned, and without access to a car. Can’t do much about that.
The more cars drive down a street, the more kids get hit by cars. Risk is approximately proportional to traffic volume. Children were more likely to get hurt on faster streets because faster streets usually have more cars to get hit by.
That result is usually cited, dishonestly, as saying speed is dangerous. If two streets are equally busy, the faster street is often safer.
Considering speed alone, the most dangerous traffic speed is in the range 25 to 30 miles per hour. Streets where drivers normally exceed 30 mph are safer. Streets with speeds under 25 mph are slightly safer than streets with speeds over 30 mph.
Apparently children or their parents are overcompensating for hazards from fast traffic. This is risk compensation in reverse. There are a number of plausible reasons. (I’ll speculate if people want to hear some.)
The study also found on street parking is dangerous. This is important because parked cars are considered a cheap traffic calming measure. Parked cars obstruct visibility. That effect seems to be more important than changes in traffic speed.
A typical application of traffic calming involves an urban collector road where traffic averages 35 mph. The goal is to get it closer to the speed limit. Toss in a speed bump, a stop sign, and some shrubs. Change the pavement markings. Have people park on street. Speed drops to 28. Now the street is a statistically more dangerous environment.
People in power don’t want to believe that slowing traffic could ever be bad or that laissez-faire could ever be good. Across America urban planners look for places where traffic exceeds 30 mph and slow cars down to make streets more dangerous for the children.
This is not driving advice. I never said that you were safer at 20 mph or 35 mph. Streets where most drivers go those speeds seem to be safer. Maybe it’s drivers going 20 or 35 where everybody else goes 30 who run over children. People aren’t good at judging the speed of individual cars. When in doubt, stay with the flow of traffic.
This is a lesson about transport policy, one more fact that everybody knows that happens to be wrong.
