You're confined to a single lane as you drive along a narrow, winding road. The car in front of you suddenly slows, then just as inexplicably accelerates a short time later, only to slow again. As you keep adjusting to the leading car's erratic speed changes, you sometimes find a clump of vehicles closely tailing you and, at other times, only a few vehicles in sight.
From a physicist's point of view, traffic flow can be regarded as a "many-body system of strongly interacting bodies." Various studies have revealed that such systems can show wavelike behavior and abrupt transitions from one state to another. The shift from freely moving to jammed traffic, for example, corresponds to a phase transition