A system (200) for controlling road traffic comprises measurement station (210) for determining air pollution in an urban area comprising pollution zones (221-223) with predetermined geographical boundaries. A central system is configured to charge a pollution fee to a vehicle (10) moving in the pollution zone when the air pollution exceeds a threshold value. Different vehicles are charged different fees, e.g. based on their function and emission. A variable message sign (240) warns of the increased fees, so most private cars choose alternative routes (232, 233) around the pollution zone (222, 223), but allow motorists willing to pay a substantial pollution fee to use route (231). Heavy diesel vehicles may be exempt from pollution fees along a direct route (231) through the pollution zone to keep them away from small roads and residential areas. A method using mobile devices for registering passengers for a passenger discount is also described.