Decisions to send an ambulance with or without lights and siren are made every day. While travel with lights and siren is presumed to have relatively more risk associated with it than travel without, little epidemiologic analysis has been conducted to compare the two modes of travel or to characterize collisions in general.

To characterize ambulance collisions and assess the risk of traveling with lights and siren in an urban 9-1-1 environment.

Retrospective analysis of all consecutive ambulance collisions of the Paramedic Division of the San Francisco Department of Public Health during a 27-month period.

The overall collision rate for lights and siren (LS) travel was higher than that for non-lights and siren travel, although the difference was not statistically significant (45.9 collisions per 100,000 LS patient travels, 95% confidence limits 29.7, 62.1, versus 27.0/100,000 for non-LS travel, 95% confidence limits 18.3, 35.7). However, the rates of resulting injuries displayed a statistically significant difference (22.2 injuries per 100,00 LS patient travel, 95% confidence limits 11.0, 33.5, versus 1.5/100,000 for non-LS travel, 95% confidence limits −0.6, 3.5). While the majority of collisions (60.0%) occurred during patient-related travel, 35.6% occurred while the ambulance was available awaiting assignment, and 4.4% in a hospital parking lot. The majority of collisions were due to inattention, failure of on-coming traffic to yield, or unsafe parking; unsafe speed was an infrequent cause. Most crashes occurred during daylight, in dry weather, and involved another vehicle.

There is some elevated risk for collision and added injury during lights and siren travel compared to travel without LS. The causes for these collisions suggest that interventions designed to improve driver skills and increase citizen awareness of an approaching ambulance could help reduce the number of collisions.