I heard that the deer whistle is ineffective. Here is an
article on it. I read that it does the opposite.
There are times that getting hit by a deer is unavoidable. Many years ago, I had a deer cross a field then a road right into the side of my Sentra. It shattered the drivers side window and broke off my left out side rear view mirror. It was night but it had left. I only saw it a few seconds before it hit my car and it was too late to avoid.