United States Highway 30 between Kemmerer and Cokeville, Wyoming bisects the migratory route of the Wyoming Range mule deer (Odocoileus hemionus) herd and is the site of hundreds of deer–vehicle collisions each year. We tested the effectiveness of the FLASH™ (Flashing Light Animal Sensing Host, Victoria Gouch, Meridian, Id.) system, designed to detect deer presence on the highway and warn motorists by triggering flashing lights associated with a sign. We collected data on changes in vehicle speed in response to the warning system and conducted a series of experimental manipulations to determine motorist response to the system with various treatments involving the sign, the lights, and the presence of a deer decoy. Motorists in automobiles traveling at night reduced their speed the most (6%) in response to the normal operation of the system, with a mean speed reduction of 6 km/hour. During experimental manipulations motorists reduced their speed 20% when the deer decoy was present in the crossing, and responded less (7%) to the flashing lights and sign. The system tested may be effective in preventing deer–vehicle collisions in areas with a lower speed limit and more local traffic, but we believe it is not suitable for an application with high-speed traffic (i. e., >100 km/hour) and a high proportion of interstate traffic.