Objective Measure the impact of automated photo speed enforcement in school zones on motorist speed and speeding violation rates during school travel.

Methods Automated enforcement cameras, active during school commuting hours, were installed around four elementary schools in Seattle, Washington, USA in 2012. We examined the effect of automated enforcement on motorist speeds and speed violation rates during the citation period (10 December 2012 to 15 January 2015) compared with the ‘warning’ period (1 November to 9 December 2012). We evaluated outcomes with an interrupted time series approach using multilevel mixed linear regression.

Results Motorist speed violation rates decreased by nearly half in the citation period compared with the warning period (standardised incident rate ratio 0.53, 95% CI 0.42 to 0.66). The hourly maximum violation speed and mean hourly speeds decreased 2.1 MPH (95% CI –2.88 to –1.39) and 1.1 MPH (95% CI –1.64 to – 0.60), respectively. The impact of automated enforcement was sustained during the second year of implementation.

Conclusion Automated photo enforcement of speed limit in school zones was effective at reducing motorist speed violations and also achieved a significant reduction in mean motorist speed.