I've been using deer whistles for 5 years and I think they do work. I used to travel a lot, especially back county roads and rural highways. Most of the time what I've experienced is when the deer is on the side of the road and in full stride, they freeze. This is from personal experience. I have not hit one (knock on wood) yet but I think placement is all the difference.
I'll second the body shop notion. My friend hit one, then put on a deer whistle, has not hit one since. Is it luck, it could. But the deer population is SE Michigan is booming. I have deer in my backyard almost every day in the fall and winter. Usually herds for 5-6. They nest in my moms back yard. Sometimes seeing the males fighting which is cool to watch.