Data were collected on an interstate highway in Illinois for two speed control treatments: police patrol car (Police)and speed radar photo enforcement (SPE) as well as the Base condition with no treatment. The data collectionscheme utilized two locations to characterize the effects near the treatment and 1.5 miles downstream. Vehicleobservations from the two locations were matched. The paired data were used to analyze the speed change behaviorof vehicles between the two data collection locations as a result of the speed control treatments, vehicle type, andlane type.Paired t-tests revealed statistically significant speed increases of 6.1 mph and 5.4 mph between thetreatment and downstream locations for Police and SPE, respectively; the speed change for Base condition was notstatistically different than 0 mph. The magnitude of average speed change of each treatment was statisticallydifferent from each of the others.Travel lane at the treatment location was not a significant factor in speed change but vehicle type(passenger car or heavy vehicle) was a significant factor. Passenger cars had a higher average speed change thanheavy vehicles for Police and SPE but lower average speed change for Base.An overwhelming percentage of drivers sped up after treatment in Police (92%) and SPE (85%) while only48% sped up in the base condition.In Police and SPE, 57% and 54% of drivers were not speeding at the treatment location but sped up suchthat they were speeding at the downstream; only 13% of the Base group showed this behavior.
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