Using terror alert levels to estimate the effect of police on crime

Changes in the terror alert level set by the Department of Homeland Security provide a shock to police presence in Washington, D. C. Using daily crime data during the period the terror alert system has been in place, we show that the level of crime decreases significantly, both statistically and economically, during high-alert periods. The decrease in the level of crime is especially large in the National Mall. This provides strong evidence of the causal effect of police on the level of crime and suggests a research strategy that can be used in other cities.