About 5 years ago I hit 2 deer in 2 months. The second one I hit did some pretty good damage. An older driver at the truck stop asked me what happened. After I told him he said deer whistles really do work.
He said most drivers put them on and leave them forever thinking they will work forever. Well they don't work forever because they get bugs and grime in them. So, I switch mine out every 3 to 6 months and haven't even had one run out in front of me for the past 5 years. I always thought deer whistles were a joke but, seems to be working.