The migratory route of the Wyoming Range mule deer (Odocoileus hemionus) herd is bisected by U.S. Highway 30 between Kemmerer and Cokeville, Wyoming, resulting in hundreds of deer-vehicle collisions at this site each year. We tested the effectiveness of the FLASH system, designed to detect deer presence on the highway and warn motorists by triggering flashing lights associated with a sign. We collected data on deer activity, system reliability, and vehicle speed in response to the warning signs. We also conducted a series of experimental manipulations to determine motorist response to the system with the lights flashing or not flashing and with the presence or absence of a realistic deer decoy in the road. It was found that more than 50 percent of the hits registered by the FLASH system were false hits not caused by deer, though a backup deer detection system worked well throughout the study period, with no false hits detected. Vehicles were not found to slow down significantly for the warning signs, although they did slow down in response to deer presence. During the experimental manipulations, vehicles only significantly reduced their speed (11.6 and 6.3 mph on average for passenger vehicles and tractor trailers respectively) when the deer decoy was in the crossing. Vehicles responded to the other treatments by reducing their speed by an average of less than 5 mph. The system tested may be effective in preventing deer-vehicle collisions under different traffic conditions, but is not suitable for this particular site.