Driving speed was measured using radar in an area posted with a speed limit of 35 miles per hour. The variables were the presence or absence of a speed limit sign, a radar enforced sign, and a marked police vehicle. Only in the presence of the marked police vehicle were systematic changes in driving speed observed. Most drivers exposed to the marked vehicle showed large reductions in driving speed. These reductions occurred regardless of whether the driver's initial speed exceeded the posted speed limit, and generally went well below the posted limit. Speed reductions in the presence of the marked car reflected an overreaction to the threat of punishment and suggest that driving speed is controlled more by external threat than by drivers' acceptance of the value of safe and energy-efficient driving. Tabular data, a diagram, and 13 references are included. (Author abstract modified).