This study examined the effects of speed photo-radar enforcement on cars and trucks speeding through interstate work zones near St. Louis and Chicago. Traffic video data collected before and after the system was installed was used to estimate impacts on average vehicle speeds and degree of speeding during off-peak hours when speeding was most prevalent. Results were compared to conditions with and without police presence to gauge overall effectiveness.
Researchers examined impacts on overall traffic speeds as well as changes to speeds by vehicle type (cars and trucks) in each travel lane. The degree of speeding was divided into four levels.
- Percentage of drivers who exceeded work zone speed limits (55 mi/hr for both sites).
- Proportion of drivers who exceeded the speed limit by 5 mi/hr or less.
- Proportion of drivers who exceeded the speed limit by 5 to 10 mi/hr.
- Proportion of drivers who exceeded the speed limit by more than 10 mi/hr.
- Traffic data extracted from video recordings showed photo-radar enforcement reduced numbers of speeding cars and trucks at work zones. Speeding cars decreased 40 to 51 percent in median lanes and 7 to 57 percent in shoulder lanes. Average car speeds decreased 6.3 to 7.9 mi/hr in median lanes and 4.1 to 7.7 mi/hr in shoulder lanes.
- Speeding trucks decreased 10 to 53 percent in median lanes and 0 to 56 percent in shoulder lanes. Average truck speeds decreased 3.4 to 6.9 mi/hr in median lanes and 4.0 to 6.1 mi/hr in shoulder lanes.
- Spatial effects were also observed. At a location 1.5 miles downstream from each work zone the total number of speeding drivers decreased 0 to 44 percent. Persistent halo effects, however, were negligible after the system was removed.
- Overall, the system was found equally effective as police patrols with emergency lights turned off.
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