<I used them for a couple of
years, and as I recall, it seemed the deer stayed motionless so you
could just go around them. Cattle, however, are immune (or deaf), but>

I wouldn't be so sure that that is a function of the whistle and not just standard deer behavior. They often stand motionless ( the "deer in the headlights" thing) and then leap at the last second. As I said earlier, I have commuted from 60 miles west of Washington, DC into the city at 0-dark-thirty for years, and have never hit a deer but have come close. Last year I passed a deer just inches from the shoulder-frozen there like a statue-and the second I passed it jumped in front of the car behind me, and was nailed. I don't have the whistles, and have never hit a deer- I know folks who do have the whistles and have never hit a deer. So, whether they work or not I don't know.     Jeff