Last Friday I was in North Wales for the UK launch of the new BMW 6-series coupe. At the press conference, it was mentioned in passing that the car gets 'BMW Professional' sat-nav, which included live traffic updates.

It would have been easy to have let that go as unremarkable, because many sat-nav systems promise live updates of traffic conditions. However, what BMW calls 'Real Time Traffic Information' is a genuine revolution.

The 6-series' sat-nav screen uses colour-coding for the main roads to indicate the speed of the traffic. Green shows that it is flowing normally. Yellow shows traffic slowing, amber means slow and red means traffic is at a standstill. It can also determine the traffic flow on opposite sides of the road.

As you can see from the screen shot that I took this lunchtime in Battersea, the system is accurate down to a few hundred feet, with slowing traffic north-bound on Battersea Bridge Road (thanks to the nearby Albert Bridge still being closed) and traffic at a standstill on west-bound Chelsea embankment, though only over a few hundred yards.

At the press conference I asked just how the system could be so accurate. The answer was amazing. Information on traffic speed is collated from information from thousands of motorist's mobile phones. It combines something called TPEG (Transport Expert Protocol Group) with real-time information from thousands of mobile phones, whose location and speed across the ground is monitored.

Roughly, it tracks groups of mobiles travelling at greater than walking speed - which are assumed to be inside a motor vehicle - along the same major roads, and monitors the speed at which the phones are moving.