I agree with the other posts, deer whistles are not effective. Just like the sirens on my patrol car, I am usually out-driving the sound they make when responding code. Studies show that our wig wag headlights are visible before the red and blues and before the siren is heard off of a long straight stretch test area. That being said, I live in the Pacific Northwest, Deer and Elk capital. Deer will stop and look before moving out of way. Instinctual. A moving target is a visible target in the animal kingdom. Elk, well they know they are the biggest things on the road and just don't care. Best way to avoid deer is to work your eyes and look for them.