The London Congestion Charge was far from universally popular when it passed into law in 2003, but despite early opposition from the Conservative Party - and even some London councils - it looks like it's here to stay.

Introduced to reduce traffic congestion in central London and raise investment to make much-needed repairs and upgrades to the city's transport system, the scheme has been largely hailed as a success. It's been reported that traffic levels in the zone have dropped by around 20%, cutting air pollution, clearing routes for buses and the emergency services, and making the roads safer for London's growing population of cyclists. Now, other large cities around the world are looking at London as a blueprint for introducing their own congestion zones.

How does the Congestion Charge work?

The Congestion Charge is a daily charge of £8 on motorists driving inside the Congestion Charge Zone between 7am and 6pm, Monday to Friday. Don't expect to see toll booths or barriers, though. To pay, you register for an account at a shop or on the Transport for London (TfL) website.

The Congestion Charge is primarily there to reduce traffic at the capital's busiest times, so it gets days off like everyone else - you don't have to pay up on weekends, English public holidays or charging days that fall between Christmas Day and New Year's Day.

Where is the Congestion Charge Zone?

The Congestion Charge Zone currently covers a good chunk of central London, reaching down to Vauxhall Bridge in the south and up to Kings Cross in the north. (Look out for signs and a white-on-red C painted on the road.)

But while there were originally plans to roll the scheme out further into the suburbs, London Mayor Boris Johnson has ruled out any further growth, and there is some speculation that he will dismantle the toll's western extension (introduced 2007) later this year. This would narrow the zone down to cover an area roughly between Buckingham Palace in the west and Tower Bridge in the east.

For drivers looking to cross London, there are also some free through routes - primarily, the route between Vauxhall Bridge Road and Edgware Road, and the Westway, which runs between Paddington and North Kensington. Visit the TfL website for a map that shows the boundaries of the zone and all through routes.

Who is exempt from the charge - and can I get a discount?
Certain vehicles, including buses, taxis, ambulances, fire engines, motorcycles, some minibuses, small three-wheelers and vehicles that run on alternative fuel are exempt from the charge - although for many of these exemptions, you will still have to register to avoid receiving a fine.

If you live in or near the Congestion Charge Zone you may be eligible for a 90% discount if you plan to pay the charge for a week or more at once. If you've been paying the Congestion Charge while waiting for your application to clear you may even be able to claim a refund on charges since applying.

If I don't pay, how will they catch me?

The system operates using a piece of surveillance technology called Automatic Number Plate Recognition. Cameras scattered around the zone read your number plate and check it off against a database to determine if you've paid or if you're exempt or have a discount. Failure to pay results in a fine of £120, which is reduced to £60 if paid within 14 days, but increases to £180 if unpaid after 28 days.

I've received a Penalty Charge Notice unfairly - how do I complain?

If you've simply forgotten to register, good luck, but that one probably won't wash. However, if you've received a Penalty Charge Notice (PCN) unfairly, you can make a challenge to TfL. There have been reports that some motorists have been cloning number plates, resulting in innocent vehicle owners receiving penalty notices when their vehicles haven't been anywhere near the zone. It's up to you to prove your case, though. And before you get any ideas, be warned - displaying false plates can earn you fines of up to £1000, and the DVLA has the power to revoke their registration number, so best not to go there.

Updated: 21/03/2011

Louis Pattison