This was on Tomorrow's world MANY years ago - they were intending to replace emergency sirens with it. I think this was partly rolled out occasionally you'll here a siren go WOO WOO krrrrr WOO WOO krrrr - the same principle applies.,(reply>), Mmm, I was 95% sure it was Tomorrow's World that was showing it.<br>Most now do yelp, wail and machine gun (or phaser). Yelp and wail are the same, but the machine gun is a faster sound (like a child making machine gun noises). It's more common for a driver to switch to that one when crossing a junction as it has a more urgent feel to it. The change in tone when about to negotiate an obstacle is meant to act as a cue as well. "I'm driving, I'm driving, I've changed pitch. I'm about to do something different."