I don't believe it's safe. Firstly a lot of people suffer from tinnitus, which is a constant hiss of white noise in their ears. How will they distinguish the sound of the trucks from that?  Secondly the white noise is similar to the sound of air discharging from air brakes used on most trucks, so it will be considered as a normal sound which trucks make, not as an alarm. I worked in a yard with reversing trucks for a while, and the reason I am still here to tell the tale is because of those old fashioned beepers, which immediately got my attention. It reminds me of the variable pitch sirens on emergency vehicles, which mimic the Doppler effect, and make it impossible to determine from which direction the sound is coming. Maybe that is why there is an increase in the number of pedestrians getting run over by police cars.