Objectives To determine if the commonly acknowledged relationships between population density and traffic collisions are found at the subcounty level and to describe how collision characteristics may vary substantially at a local level, with a particular emphasis on exurban areas.

Design Los Angeles County collision data were obtained from the California Highway Patrol and the census tract and service planning area (SPA) for each collision were determined. The correlation between population density and collision rates by census tract was calculated within each SPA and for the entire county. Primary collision factors were compared for geographic areas of different population densities within one exurban SPA in Los Angeles County.

Results An inverse relationship was found between collision rates and population density within Los Angeles County. Primary collisions factors were different in areas of the county with different population densities, with driving or biking under the influence particularly common in the most rural area.

Conclusions Subcounty analyses are very important to the study of traffic collisions. Traffic problems in rapidly developing exurban areas may be quite different from those in older, more established areas.