You may have seen them, a pair of small whistles that you attach to your vehicle to scare deer away and prevent collisions with them. The intent is for the movement of your vehicle to force air through them and produce sound, or ultrasound that keeps deer off the highway and safely out of your path. The answer hasn't changed from the first time I wrote about this in the 1990's, keep your money in your pocket, they don't work.
The average deer has the same hearing range that humans do. This means that like you, the deer cannot hear ultrasonic whistles. It doesn't take much thought to decide just how effective these will be.
Audible whistles emit sound around 3 and 12 kHz depending on whether they are open or closed ended. Tests conducted at the roadside found that the noise from the whistles was not significantly louder than the road noise created by the vehicle. If other noise is present in the environment the whistles can be even less noticeable.
Oddly enough, in a study that observed the reaction of deer to vehicle with and without deer whistles actually found that more deer paid attention to vehicles without the whistles than those equipped with them. The conclusion appears to be that drivers would be better to save their money, slow down and drive carefully where deer are known to be a hazard.