What are the hopes for congestion charges?
London's mayor, Ken Livingstone, hopes making drivers pay to enter the city centre will cut traffic by 10%.

As well as improving the lot of the city's pedestrians, cyclists and bus drivers, those motorists who pay the charge will find their journey times much reduced, it is argued.

Reduced noise, air pollution and traffic accidents would also pay economic as well as social dividends.

Where will the toll zones operate?
In London, the heart of the city will be designated as the charging area. Drivers will pay to venture inside a ring road - running from Victoria station in the west, to Fenchurch Street station (east), Elephant and Castle (south) and Kings Cross (north).

Cities across the UK are considering similar schemes. Belfast, Birmingham, Durham, Leeds and Bristol are all pondering tolls as a way to reduce congestion at peak times.

Figures mooted for other schemes include a �2 morning rush hour fee for Belfast and a �3.50 daily charge for Birmingham and Wolverhampton.

Who won't have to pay?
Motorcyclists will be exempt from the London toll, as will those drivers of licensed taxis, disabled or "green" vehicles.

Drivers living within the charging zone will qualify for a 90% discount and residents' parking will be totally exempt. Concessions will also be available for low-paid education and hospital workers living or employed within the zone.

Charges will be dropped for everyone on Christmas Day and Bank Holidays.

How will the money be collected?
London drivers will be able to purchase daily, weekly (�25), monthly (�110) or annual (�1,250) passes by post, the phone, online, or in newsagents, shops or garages.

How will the charges be enforced?
The registration numbers of those paying will be entered into a database. Fixed and mobile cameras trained on the number plates of cars entering central London will search out non-complying vehicles.

Such automatic number plate recognition (ANPR) systems arrived in the UK in 1997, employed by the City of London police to address the risk of terrorist attacks.

Wardens will patrol parking bays to locate those non-payers who don't actual hit the roads.

Is this the only system for city tolls?
Singapore employs a scheme whereby drivers pay money into an account from which the toll fee is deducted automatically when the "smartcard" tag passes a roadside sensor in the restricted area.

Oslo installed automated booths (coin and card operated) to harvest tolls, though Norwegian commuters were also encouraged to buy smartcards.

How much will toll evaders be fined?
The London scheme will track down the owners of non-complying vehicles via their registration plates.

A flat fine of �80 will be levied, but a discount of �40 will be rewarded for prompt payment. The penalty will be increased to �120 if the fine is not paid in reasonable time.

Where will the excess cars go?
Ken Livingstone hopes 200 more buses a year in the run up to the charge scheme's introduction in late 2002 will offer motorists an alternative to driving.

Despite this and other promised public transport improvements, the mayor admits some roads outside the toll zone will see increased traffic.

How much do such schemes cost?
The Oslo toll zone sees revenues five times higher than the scheme's operating costs.

The London project, estimated to cost �200m to introduce and �50m a year to run, is estimated to reap as much as �200m a year.