Effects of public posting on driving speed in Icelandic traffic.

We replicated a study by Van Houten, Nau, and Marini (1980) that had revealed reductions in vehicle speeding following the posting of percentages of drivers not speeding on a sign at roadside. Our subjects were drivers entering a residential area where the speed limit changed from 90 km/hr (55.9 mph) to 60 km/hr (37.3 mph). A total of 4,409 vehicle speeds were taken from two observation sessions per day for 20 consecutive weekdays. The intervention consisted of a single posting condition, in which a hypothetical daily percentage of drivers not speeding was posted on a feedback sign, followed by a double posting condition, in which a sign posting a best result was erected beyond the feedback sign. Results revealed a significant speed reduction from an average of 69.0 km/hr (42.9 mph) during baseline to 63.4 km/hr (39.4 mph) during single posting. Average speed during double posting was 62.9 km/hr (39.1 mph). The percentage of drivers exceeding 70 km/hr (43.5 mph) dropped from 41.0 during baseline to 20.5 during single posting. The significant speed reductions add to the generality of findings of similar studies in Canada and Israel and offer possible explanations for the failure of feedback posting to reduce speed in the U.S.