University of Geogia Study
Researchers at the University of Georgia concluded that deer whistles mounted on cars did not produce ultrasonic sound that was audible to deer, although low frequencies were produced when the whistle was blown orally. The Georgia Game and Fish Department found that after observing several hundred incidents in which cars carrying deer whistles approached deer, they did not react.
University of Wisconsin
A study conducted by the University of Wisconsin concluded that deer did not respond to the whistles, even when drivers were well within the quarter-mile limit designated by manufacturers. Anything blocking the signal of the whistle that stood between it and the deer would also impede the deer from receiving it. According to the study, the whistle did produce an ultrasonic sound at speeds of more than 30 mph, but there was no evidence indicating that the deer heard it.
Farm Journal
A 1993 Farm Journal article indicated that Ohio State Police had little success when installing deer whistles on their vehicles, as there was no measurable decrease in the number of collisions with deer and quoted a study conducted by World Society of the Protection of Animals as saying that the whistles were ineffective when keeping deer or other animals from traversing a highway, rendering the whistles useless.