Discretion and Ulterior Motives in Traffic Stops: The Detection of Other Crimes and the Revenue from Tickets∗

In the United States, police officers often decide to give drivers they stop for traffic violations a warning, which imposes no fine, instead of a ticket. Officers are also legally permitted to stop drivers for the purpose of detecting other crimes. This paper addresses two questions about the role of discretion and ulterior motives in traffic stops. First, under what conditions may it be efficient to let many stopped drivers go with only a warning? Using a model of law enforcement based on Shavell (1991), who does not consider warnings, I show that the ulterior motive of detecting other crimes is a simple way to rationalize the existence of warnings in an efficient enforcement scheme. Second, I test the model against data on traffic tickets and warnings in Massachusetts to determine whether police discriminate against out-of-town drivers because of the ulterior motive of ticket revenue. I find support for the notion that discrimination against out-of-town drivers is motivated by revenue. In the model, the revenue motive is an aspect of efficient enforcement for a local government. ∗I am grateful to Paul Courant, Jim Hines, Stephan Lindner, Yusufcan Masatlioglu, J.J. Prescott, Paul Rothstein, Matt Rutledge, Dave Schmidt, Dan Silverman, Doug Smith, Jeff Smith, and seminar participants at the University of Michigan, and the Association for Public Economic Theory’s 2009 meeting for many helpful comments. I thank Kate Antonovics, Bill Dedman, and Nicola Persico for sharing data. †The views expressed in this article are those of the author and do not necessarily reflect those of the Federal Trade Commission.