This paper uses the crash records, which are part of the Highway Safety Information System (HSIS), for California highway system from 1999 and 2000 to identify the prevailing characteristics of secondary crashes for the aforementioned system. The crash records are aggregated to the California Department of Transportation (Caltrans) District level for analysis. The study compares differences in the characteristics of secondary crashes and primary crashes with respect to time of day, roadway classification, primary collision factors, severity level and type of accident. This study defines a secondary crash, which is any crash that results from the non-recurring congestion or emergency response associated with a primary crash, as any crash that occurs in the same direction within sixty minutes of a primary crash and no more than two miles upstream.

For all comparisons, a proportional test is used to assess the presence of significant differences between secondary crashes and primary crashes. One such comparison is a comparison between rural and urban Caltrans Districts. As expected, a higher proportion of crashes in urban Districts are classified as secondary crashes than in rural Districts. The typical secondary crash on the State of California Highway System is a rear-end, property damage only crash on a greater than a four lane urban freeway that occurs during one of the peak periods and is caused by excessive speed.