By Tom Symonds
BBC transport correspondent
Road pricing is the government's radical proposal to cut congestion - but how much could it cost?
Road charging has one simple principle. You pay more to use the roads when they are busy, or more congested.
The BBC's Pay As You Drive road-charging trial monitors the hourly time period in which the vehicle is moving, the type of urban area it is passing through, the road type and, of course, the distance travelled.
We have enlisted the help of Professor Stephen Glaister, one of the country's top transport economists, based at London's Imperial College.
Rural A-road: 5p a mile
Motorway journey: 13p a mile
Big city back streets: 11p a mile
Busy city A-roads: 27p a mile
He has created a computer model which sets prices per mile of road, with the aim of reducing traffic congestion.
The prices are calculated using assumptions for the cost of pollution, time lost through traffic congestion, and accidents caused by heavier traffic.
The examples on the right are based on an average area of the country - the Midlands.
Rural A-road: Free
Motorway journey: Free
Big city back streets: 3p a mile
Busy city A-roads: 6p a mile
A quiet Sunday drive through country lanes: Free
Charges for some areas such as the South East and London would be higher. The government has said the highest
possible price could be as much as £1.30 per mile.
And although in our examples motorway driving during off-peak hours is free, it is thought ministers may introduce charges.
These prices would result in drivers paying more for using roads. But it is possible the government will get rid of fuel duty and the car tax disc, which might also cut the burden on motorists.