This question comes up a lot...a bit long. I ride and some of the guys I ride with just add them to their bike for extra chrome\bling. :biggrin:
From Wikipedia : Scientific studies of these devices have indicated that they do not in fact reduce collisions.
Researchers with the Georgia Game and Fish Department and University of Wisconsin-Madison have pointed out several reasons why these devices do not (and probably could not) work as advertised:
Some deer whistles do not emit any ultrasonic sound under the advertised operating conditions [typically when the vehicle exceeds 30 mph].
Ultrasonic sound does not carry very well. It does not travel a long enough distance to provide adequate warning, and also is stopped by virtually any intervening object, so any curves in a road will block the sound.
We know little about the auditory limits of deer, but what we do know indicates that deer hear approximately the same frequencies as humans, and thus if we can't hear it, they probably can't either.
If deer could hear ultrasound, we do not know that it would alarm them or induce a flight response. Certainly if they heard it on a regular basis, they would get used to the sound and not react to it.