A congestion charge makes drivers pay to enter a city centre at busy times. Traffic falls immediately when one is introduced, typically by ten to twenty per cent, and buses inside the zone move faster because the road ahead is clearer. The effect on drivers is uneven, however: those who can shift their journey to another hour or another route do so, while those whose work requires them to be in the centre at nine simply pay. A charge, in this sense, sorts journeys by flexibility rather than by importance.