How do speed cameras cause traffic jams?
It’s all about standing waves. Derek Turner, the government’s car tsar, has the unenviable task of clearing Britain’s clogged roads. He said this week that speed cameras worsened traffic congestion. This might sound obvious. Slower traffic means it’ll take longer to get where you’re going, right? “It’s not true that slowing traffic down causes traffic jams in itself,” says Stephen Glaister of the Centre for Transport Studies at Imperial College. “It’s the behaviour of motorists in the vicinity of traffic cameras, because they suddenly jam their brakes on, that causes the problem.” Sudden braking, to avoid a camera or a police car, can cause problems. Drivers are familiar with this: they come to a halt in fast-moving traffic, sit for a while and then move off again, for no apparent reason. This is a standing wave. While there is little doubt that speed cameras are good at forcing drivers to obey speed limits, their location can exacerbate traffic problems. “They tend to be placed where