Data collected in Maryland covered speed distributions in the presence and absence of conventional police radar as measured by an automatic speed monitoring station, speed distributions in the presence or absence of police radar as measured by nondetectable radar, and speed changes among fast vehicles as measured by nondetectable radar when police radar was suddenly activated. Data collection procedures were similar in Virginia except that speed distributions from monitoring stations were not collected. Findings indicate that overall travel speeds are lower and the proportion of vehicles traveling at very high speeds are lower when hidden police radar units are operating. Tractor trailers showed the greatest declines in average speed and in the proportion exceeding 65 mph in the presence of continuously operating detectable police radar. Results suggest that radar detectors are being widely used to elude police speed enforcement and that travel speeds on U.S. highways are faster than they would be in the absence of radar detectors. 3 tables, 1 figure and 7 references.