Of all the things/gadgets that I've tried and have confidence with, this is (ultrasonic deer whistles, anti-deer warning device, whatever you like to call it) something that I'm never too sure about.
That being said, however, I had my fair share of encounterings with deers or animals (in general) while travelling along the highways. I had installed a pair of Superex deer device and it seems to work (this is just a subjective observation) throughout my 5+ years of living 1000kms away from the metro....deers would be completely look "dazed" if in your headlights in the middle of the night but with deer whistles installed" they dazed for a second or 2 before they would either hop over to the other side of the road or "retreat".......
I had schoolmates that hit a deer on highway before; I've seen truckers hitting something big on the highway also (gauging by the dried blood trace on the stretch of highway...
I think the air-driven type may work better than those electronic ones...but then again: that's just a guess...
Q.