I've been driving outback for 40 years and had a few hits, plenty of 'close ones' and for the last decade I splashed out with 6 bucks at supercheap for a pair of black plastic whistles and my experience is they work. Will they save you from ever hitting a roo again, No, but if they reduce the incidence by my guess 80% well $6 well spent. I've mentioned it to many people and some say BS but others have agreed in a change in the behaviour pattern.