Personal experience here. Frequently my wife and I ride over a 6,000+ foot elevation pass on a state highway when we're headed home after long weekend rides. At dusk and dark it is swarming with deer. Of course, we take it easy and use caution. The reason for all the deer is this. The area is a winter recreational area and heavily visited during that time and parking during that time is at a premium. Because of that the highway department salts and sands the area heavily. During the summer evenings the deer flock to that area to lick up the salt. My experience riding with both types of deer whistles on motorcycles and cars in that area had zero affect. Just about the same as using a horn. They do nothing until you're passed them. The whistles are supposed to be in a tone range that humans can't hear, only animals can hear that tone. How do you know your whistles are working, well you don't. I have passed many deer standing alongside the road and mostly they are just standing there or they are grazing and paying no attention to traffic. Those that get hit are on a panic run or just standing in the way and the driver simply didn't see them due to the lack of attention. As others have mentioned here, be cautious and aware when in a deer area. Slow down at night and look for the deer’s eyes and don’t out run your headlights.