Shifty Business

Lots of years ago, ambulances and other emergency vehicles in the UK switched from using two-tone sirens to the wail/yelp type they have now.

The reason given was that the public were more accustomed to this type of warning, since it was what emergency vehicles on TV used – ironic, since the reason TV companies used these alternative sirens was to avoid confusion with real emergency vehicles! Anyway, there was much comment at the time that people found it difficult to tell where the new-style sirens were coming from.

I was reminded of this the other day: I was driving home from work, and heard an ambulance siren – I’d already seen the ambulance coming towards me, and there was nothing I needed to do in terms of getting out of his way, so I carried on in my own sweet way.

Then it dawned on me that the ambulance I’d seen didn’t have blue lights flashing, and that was when I checked all around properly: sure enough, the ambulance with the siren was approaching from behind. I’d seen it in plenty of time to get out of its way, but being unable to tell as soon as I heard it cost a couple of seconds reaction time which in heavier traffic might have made a difference. It isn’t just me, and it isn’t about driving skill – the comments I referred to above were on the Institute of Advanced Motorists discussion forum.

I think it’s something to do with the way the ears use doppler shift to help determine where a sound is coming from: the constantly-changing note of the new-style sirens confuses the hearing sense, which is expecting a steady note with overlaid doppler. The result is either that the sound seems to come from all around – like out-of-phase stereo – or from a false direction.

If any emergency drivers are reading this – and I’m fairly sure there’s at least one among my regular readers – I’d be interested in how common you think it is for people to misjudge where you are.

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