The geography of deterrence: exploring the small area effects of sobriety checkpoints on alcohol-impaired collision rates within a city.

This article examines alcohol-impaired collision metrics around nine sobriety checkpoint locations in Indianapolis, Indiana, before and after implementation of 22 checkpoints, using a pre/post examination, a pre/post nonequivalent comparison group analysis, and an interrupted time series approach. Traffic safety officials used geographical information system (GIS) analysis to help select checkpoint locations amid high alcohol-related collision clusters, then analyzed possible checkpoint impacts. A post hoc analysis examined counts and rates of impaired collisions before and after checkpoint dates within the 2-mile radius zones around each checkpoint site and compared pre-to-post differences to two similar time-matched control areas without checkpoints. As a group, checkpoint zones showed significant though minor declines in pre-to-post collision counts, and no impairment rate change. Considered together, non-downtown checkpoint zones had more favorable impairment rates than comparison areas. After controlling for collision volume, monthly trend, and locational effects, the interrupted time series analysis found that for all checkpoints the count of impaired collisions in post-checkpoint periods was about 19% less than pre-checkpoint counts.