“It's not easy being green.”
The London congestion charge is a scheme implemented by the former Czar of London, Ken Livingstone (before his position was usurped by Marquise Boris Johnson Lord of Horizontal Jogging and Chief Viceroy of Pompous Buffoonery) as a new and interesting way of collecting public money for the greater good. It is loosely based on the principle of prostitution, which charges the customer to get inside a dirty hole.
The scheme was implemented with the stated goal of reducing traffic (and thus carbon emissions, which are Killing the Planet) in Central London. It has done neither. But nothing argues for expanding a government program better than unconditional success--except total failure; and the Mayor thus extended the scheme eastward in a bid to cut yet more traffic by zero, like the ice-cream vendor who realizes he's losing a few pence on every cone but thinks he'll make it up in volume.
The congestion charge is simply a set of CBBC TV cameras installed strategically around every road, including the end of one-way streets, at the edge of Central London. It records every registration number and checks it against records to see if the person inside has paid to have his car in the congestion zone. It is common for a man to jump in front of the TV when he opens his mail claiming that he has not paid the congestion charge. Of course, said man has had to pay the government simply for the privilege of having the TV.
edit From the citizen's viewpoint
Granting government any new power, such as to ring a zone with spy cameras, usually makes the citizen wonder about potential abuses of the power. The worst such abuse would be if the government studied you individually to see how much more cash it can wring out of your pockets. As this abuse is in fact the program goal, citizen acceptance was immediate and enthusiastic. After all, "we've got to do something soon," otherwise "it will be too late!"
edit From the Czar's viewpoint
Politicians don't survive by saving the planet, least of all by building clean factories, until the only way to make a tin opener cheaply is to do the work in a smog-belching factory in Kuala Lumpur. Politicians survive by ruining their opposition. And the mayor now has a spiffy database on exactly where each member of his opposition has driven and when. Although the data will never be used for such a purpose. There are safeguards in place.
Of course, it helps a politician to be able to claim he is saving the planet--and who's to know, regarding that or easing congestion? It also helps to have new buckets of new money sloshing around. Given the mischief authored by the City of London, the reader may decide whether it's wise to feed it more money on any pretense.