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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.

Summary statistics of this type can be misleading, so the authors used some fancy econometrics, the details of which I won’t bore you with. The upshot is that, controlling for other factors, for each one percent drop in the number of state troopers, road deaths on highways outside of cities (where state troopers do most of their work) rose about 0.32 percent, incapacitating injuries rose about 0.23 percent, and visible injuries rose about 0.26 percent