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.

If a group of mobiles suddenly start to slow down en masse, it is assumed that the traffic flow is slowing. The speed of the traffic is categorised in four stages and the information is updated every three minutes. The 6-series has its own built-in SIM card to receive the live information over the mobile network. From my experience this morning, it is such an accurate system that it can even distinguish that traffic approaching a major urban junction is at a standstill (see the colour banding near the SW10 script), but traffic that has crossed the junction is speeding up for a few hundred feet, before slowing to a halt again.

Yesterday evening, I was driving down the M6 from Lancashire and was warned of long delays between J19 and 21 by the matrix signs, which I usually ignore for their inaccuracy. But, sure enough, that south-bound stretch of the M6 was shown as bright red on the sat-nav.

I diverted off at Warrington and went across country to rejoin at Knutsford services, where the jam was dispersing. This system is an incredible step-forward in congestion avoidance. My only complaint is that I could do with a very quick short cut on the sat-nav, which allows me to punch in junction numbers on motorways, for easy diversions. Picking my way through Warrington and unlit countryside was not straightforward.

Otherwise, it is a surprise to realise that the mobile phone networks are selling real-time information about the location of our phones. It may be anonymised, but the idea is still a little disturbing.