There are three key types of congestion charging: area charge, as in London, where you pay when you drive in a zone; cordon charge, such as in Norway, where you pay to drive through an invisible line around an area; and wider area, normally national charging

· Green light zones
(Schemes already operating)

Singapore: electronic toll cordon around city centre, based on paper scheme introduced in 1975

London: world's biggest scheme started February 2003

Durham: tiny city-centre scheme, started October 2002

Rome: ancient city centre scheme, started late 2001, extension being considered

Norway: up to 20 towns and cities, including Oslo, Bergen, Trondheim, have cordon tolls

Melbourne: tolls on major routes into city

Austria and Switzerland: national lorry charging

· Amber light zone
(planning/seriously considering schemes)

Edinburgh: referendum on plan to start toll in 2005, linked to train and tram improvements

Cardiff: tendering for toll scheme linked to infrastructure improvements

Stockholm: pilot due to start in 2005 before referendum on keeping tolls

New York: tolls exist on many tunnels and bridges on to Manhattan island, but wider charging opposed

San Francisco: most advanced thinking in US, interest in charge zone linked to existing HOT (high-occupancy toll) lanes on main roads into city

São Paulo: looking at cordon to replace current scheme alternating vehicle access according to number plates

United Kingdom: national lorry charge planned for later this decade and consulting on scheme for all cars

· Red light zones
(traffic schemes discussed, but not yet approved)

Bristol: much-talked-of scheme on hold after Labour-controlled council lost to Liberal Democrats last year

Germany: less interest than expected, probably due to good infrastructure and public transport

Shanghai and Beijing: experts predict need for traffic controls soon because of explosion in traffic