Eric A. Morris at the Freakonomics Blog tries to answer the following: “The highway patrol exists for a reason: theoretically, by deterring dangerous driving, it’s supposed to keep us safer on the roadways. Does it?”
Attempts to measure the effects of adding substantial numbers of police or state troopers (most common in election years) or reducing the headcount (most common during budget distress). It is hard to tease out any correlations, but the thought process is fascinating.
But once in a while we get a second-best opportunity, when a city (or state) voluntarily makes a substantial change in the size of its police force, giving us a chance to see what then happened. This is called a “natural experiment,” and one took place in Oregon in 2003.
In that year, the Oregon Legislature mandated an approximately 35 percent cutback in state highway patrolmen for budget reasons. What effect did this have on accident rates?
DeAngelo and Hansen report that in the three years after the layoffs, statewide traffic fatalities rose by 19 percent, incapacitating injuries rose by 14 percent, and visible injuries rose by 12 percent compared to the three year period before. This is especially striking given that, nationwide, the fatality rate per vehicle mile traveled actually fell by 3.7 percent during the period under study.
Other characteristics that might have to do with accident rates—miles driven, precipitation, snowfall, and the number of young drivers on the roads—stayed about the same before and after the layoffs.
