I've never heard of this on lorries, but I do remember something about the white noise development at Leeds University and I recall they were trialling emergency sirens in Leeds with the white noise as part of the siren.  I was in Leeds about ten years or so ago when I came across an ambulance with this type of siren and I have to say, it really did work.  Normally when you hear a siren, you're looking around for where it's coming from, but this one I could tell was coming from behind me before I even looked.  I'm not sure why they never caught on.,(reply>), @Robert Pruitt I'm not very far off 50 myself and I'll be honest, I barely heard it too.  Mind, that could be because the camera wasn't actually on the building site, or maybe it didn't pick it up that well, because I can usually pick up white noise.,(reply>), I'm almost 50 and like millions of people, I have a bit of trouble hearing, with certain frequencies being much harder to hear.<br><br>But my hearing isn't even close to bad enough to be considered a disability. But I just barely heard that white noise, and I was focusing on it. With road or construction noise going on I'd never hear it. But I heard that beeper plenty loud and clear.