I work in public transport in a safety role and this comes up for debate every now and then. I know the theory and it is sound (there was an experiment to intermix white noise with Police sirens, but despite it working it never came to much) but in practice I don't find it a very good system at all. Too many other emissions of white noise in a modern environment and you can't hear it. In our local bus stations there are a series of air conditioning and ventilation fans and the noise they give off drowns the WN reverse alarms, but not the bleepers.<br><br>Of course, some have bleepers with several different tones and others shout out "this vehicle is reversing. This vehicle is reversing..."