The "congestion charge" was introduced by the mayor of London, Ken Livingstone, as an incentive to use our already stretched and overcrowded public transport networks.
He established a border around central london and the "West End" and levied a fee of £8 (about $12) per day for anyone wanting to enter that area using a car/van/lorry (motorcyclists, bicycles, hybrid/electric/diplomates vehicle cars are exempt - hence why I blat into town on bike).
It has reduced the amount of cars in the area, although the number of scooter riders has gone up dramatically.
T'would appear that the US & Geeermeeeen Embassy's are throwing a hissy fit, when all the other countries pay.
I go past it every day (the US embassy), and it is in a prime bit of real estate, surrounded by concrete barriers now, with fences, armed police unit, personal scanners and the whole kit and kaboodle.
Their stance over this does get peoples backs up.