A large literature considers the impact of police manpower on crime, generally finding that additional police have important albeit modest effects on criminal activity in U.S. cities.  In particular, the consensus in the literature is that police have an appreciable effect on murder, robbery and motor vehicle theft and, to a lesser degree, burglary.  However, crime reduction is only one of many roles with which police departments are tasked.  Increasingly, maintaining road safety and responding to and investigating traffic accidents is one of the most important roles that state, county and municipal police departments serve.  With the exception of a recent paper by D’Angelo and Hansen that reviews a natural experiment involving a mass layoff of state troopers, the literature has largely been silent on the effect of traffic enforcement on public safety.

Using data on police manpower from the FBI's Uniform Crime Reports and the U.S. Census' Annual Survey of Government Employment and data on traffic fatalities from the National Highway Traffic Safety Administration, we examine the degree to which increases in police strength have served to reduce the number of fatal traffic accidents in the United States.  Our data covers the period from 1975-2011.  Models are estimated at the city- and county-level and condition on a variety of fixed effects.  We also account for measurement errors in police force strength using a procedure suggested in a recent paper by Chalfin and McCrary.

 Preliminary evidence suggests that increases in municipal, county and state police manpower do, in fact, lead to a reduction in fatal traffic accidents.  The effects are economically large and are comparable to the effect of police on murder.  Thus our paper suggests an alternative and underappreciated role that police play in maintaining public safety.  An implication of our analysis is that cost-benefit analyses of police hiring that consider only the role of police in reducing crime dramatically underestimate the social benefit of police. 

 Finally, in a series of auxiliary analyses, we leverage individual-level accident data to study the types of fatal accidents that are most sensitive to police presence.  Variables studied include the day and time of accidents and the type of roads upon which fatal accidents occur as well as the characteristics of the at-fault driver and accident victims.