Here's a bit of info from Lancer's link.
whistles that attach to vehicles have
been available for more than 20
years. The whistles produce ultrasonic
noise (16 to 20 kHz) when a
vehicle exceeds about 30 mph. The
presumption is that deer will hear
the noise and be warned away. Itís
unclear whether deer do hear the
noise, but in any event studies
show the whistles have no effect on
ďPeople approach this hoping
to find quick and easy solutions,
but there arenít any. Whistles donít
work,Ē Hedlund says
Based on our research at Texas A&M, and the studies conducted by Dr.
Marchington at the University of Georgia, it seems very unlikely that
deer whistles will be effective at reducing deer-vehicle accidents.
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."