From a friend of a friend. I don't know but I'm fairly certain it's reliable:

"I've just spoken to my mum who has been at a meeting where the chief executive of the Manchester ambulance service circulated information they had received from their colleagues in London.

It appears that for gangs the current initiation test, particularly in South London, entails driving round with no lights and then the following the first person who indicates to them in any way - flashing, beeping, pointing etc. They are then to approach the vehicle and fire indiscriminately into it.

The ambulance services in London have apparently been given instruction to ignore anyone driving without lights, and, well, if they're following those precautions, I thought I'd pass it on to you too."
I see the National Guard in New Orleans have been issued with a shoot to kill policy today. With stories of rape, murder and car-jackings coming out of the city and even out of the superdome itself you can't help but breathe out a little.

Update:

Essuu has pointed out that there is an entry at Snopes.com detailing this as an urban legend. (the London gangs, not the National Guard (tho I wouldn't be surprised if some thought the National Guard was an urban legend too))