Study objective: To determine whether the use of warning lights and sirens saves a significant amount of time for ambulances responding to requests for medical aid in an urban, 2-tiered EMS setting. Methods: A prospective design was used to determine run times for ambulances responding to calls with lights and sirens (Code 3), and for a similarly equipped 'chase' vehicle traveling to the same call on the same route without lights and sirens (Code 2). Data were collected for run times, distance traveled, visibility, road surface condition, time of day, and day of the week. Simple statistics and analysis of variance were used to test for significant differences between Code 3 and Code 2 operation, as well as the other variables listed above. Results: Sixty-four runs were timed during a 9-month period. The average Code 3 response interval was 4.46 minutes. The average Code 2 response interval was 7.48 minutes. The 3.02 minutes saved on average represents a significant time savings of 38.5% (P< .01). Run distance was the only variable that was statistically significant in affecting time saved during a Code 3 response. Conclusion: Code 3 operation by EMS personnel in an urban, 2-tiered EMS setting saved significant time over Code 2 operation when traveling to a call.