From
EB1677 - "Animal Warning Devices" by Leonard Askham:
"A scientific advisory panel from the World Society for the Protection of Animals states, after extensive review, that there is no known data 'that shows that such devices can actually stop an animal crossing the road, which is the main purpose of the device.'"
"The state police in Ohio, after months of testing, found no significant decrease in patrol car/deer accidents after the warning devices were installed. In fact, more accidents were reported by the officers after the whistles were installed than before for the same period of time and stretches of highway. Tests conducted in Utah, Georgia and Wisconsin also concluded that deer whistles don't work."