Basically, Chris, it's a way to try to stop (or deter) people from taking their own cars into the centre of cities - in this case, London - congestion charging began on Monday- and to encourage the greater use of public transport.
I think I'm right in saying there are several zones - I think there are 2 proposed for Edinburgh where I live, but there are possibly more in London? - the areas are 'ringed' and depending on where you go, you are charged accordingly. It's £5 per day to enter the zone and very little is excepted. So expect a hike in goods, taxi fares, courier charges etc as companies pass on the additional charge to us poor punters!
Jackie
www.iqps.org
Peer Moderator 
