Comparative study of speed reduction effects of photo-radar and speed display boards

Two forms of automated motor-vehicle speed control, speed display boards and photo-radar, are compared. Despite a growing body of research on the devices, there is little reliable empirical evidence about their effectiveness. Three issues are examined: (a) which of these devices is more effective in lowering speeds, (b) whether supplementing display boards with police enforcement makes them more effective, and (c) which device is more cost-effective. The study was conducted on three comparable streets in Riverside, California, over a 4-week period. Site 1 employed a display board with no enforcement; Site 2, a display board with intermittent enforcement; and Site 3, photo-radar. Results show that both devices, while deployed, significantly reduce vehicle speeds 7 to 8 km/h, and particularly reduce the number of vehicles traveling 16 km/h (10 mph) or more over the posted limit. Supplementing the display board with intermittent enforcement significantly increased its effectiveness. Although both devices produced substantial speed reductions while in operation, only display boards demonstrated carryover effects. The enforced display board produced a substantial short-term (but not longer-term) carryover effect; the unenforced display board demonstrated a longer-term (but not short-term) carryover effect, but only at the alongside location, 1 week after its removal. The three cost-effectiveness estimates generated showed that the unenforced speed display board was the most cost-effective; the enforced display board came in second; and the photo-radar placed third.