This is definitely a deer deterrent - never mind the fact that I have yet to see a deer, I am sure this thing is alerting them miles away to stay clear of the road! However, as the direction booklet says, I very well may encounter a deaf deer, thus negating the 100% effectiveness that I have enjoyed thus far. Will keep you posted - hopefully they can test the deer's hearing posthumously so that I can know for certain...I don't write a lot of reviews, but I wanted to share an anecdote from the use of this product.I purchased them to install on my car ahead of a 900 mile trip, through the trees of PA, VA, WV, and VA. On two occasions during this trip did deer appear on the side of the road, only to completely change direction and sprint away from the road upon my approach. I drive an EV, so the car wasn't making engine noise.Was it the sound of the tires on the road? Was it the somewhat silent sound of the EV motor? Was it these whistles? I'm willing to spend the $8 to risk it being the latter. They're also small enough to not even be seen on the car. Easy decision, and I'll probably order more of them for the rest of the family.I live in Wisconsin (that alone should say enough) and we have deer literally walking down city streets. I whole heatedly believe in these two pieces of plastic and I will explainThe 97 Ford Exploder had whistlers on it and I had driven many days and nights from the city to the country and back. Never hit a deer however, in the middle of the dark has a random ass horse run full tilt at me down a country road but stopped right in front of me.. 2015 I got a 2010 Hyundai Elantra and drove that the same way and then wooded hills outside or hesrtford for 2 years. Sometime last year I lost a whistler. On the way to work going into a town through a part wooded and suburbs a deer ran out and hit me at my mirror in the morning. Maybe it was a coincidence. But all those years and the one year I lose it? I've had deer stand on the side of the road and stare at me going 35-45mph and not move with whistlers on.Got a new vehicle. Needless to say I'm not taking another chance.Can’t vouch for effectiveness in deterring deer but saw lots of them stopping and staring at me. I was worried about them staying attached as my car does not have the best attachment sites in the grill. Cleaned the sites with rubbing alcohol, dried, then pressed these on using the attached stickers. Was really surprised to find them still attached 1600 miles later.Encountered a deer on 2 occasions, seems to work as described.Both appeared to be aware of my approaching car and moved away very quickly...Just buy them, they are cheap. If you live in the country or around wildlife, this will save your vehicle. I've never hit an animal or deer with these on until I bought a new truck. Hit a deer the 2nd day of owning it. New truck got these on it instantly. Been going strong since. The deer run away or stay a good distance once they are alerted from a distance. It's cheap. Just get them.Time will tell if they stick and hopefully help save some deer from dying.Not sure about repelling deer but so far have not seen deer on the roads I travel