Washington, D.C., implemented a speed camera enforcement program in 2001. Vehicle speeds were measured before and after the program. Seven sites in Washington were selected randomly from a total of 60 targeted enforcement zones. Speed data were collected 1 year before enforcement and approximately 6 months after enforcement began. Results indicated that mean traffic speeds and the proportion of drivers traveling more than 10 mph above the speed limit--fast enough to warrant a speeding ticket--were reduced at each of the Washington study sites. At eight comparison sites in nearby Baltimore, Maryland, where speed camera enforcement was not in place, no decline in traffic speeds was observed. Overall, mean speeds at Washington sites declined by a statistically significant 14% compared with Baltimore sites, and the proportion of vehicles exceeding the speed limit by more than 10 mph declined 82%. These findings indicate that speed camera enforcement reduced speeding on surface streets throughout Washington. On the basis of prior research, such speed reductions can be expected to reduce the frequency and severity of motor vehicle crashes.