Most traffic jams have an obvious cause. Sometimes it is a blocked lane or a bottleneck on a multi-lane highway. An exit or entry lane can disrupt the flow as cars slow down to leave or to let others onto the road. A hill can bring things grinding to a halt behind a slow-moving lorry. Now a Japanese physicist reports in the journal Physical Review E1 that a jam can also result from a more insidious factor: a single driver whose speed fluctuates unpredictably.

Computer models are now used to predict the traffic flow pattern in some cities, with data from a few monitoring stations. In a typical traffic model, each vehicle tries to accelerate to a certain desired speed, and modifies its speed in response to the vehicles ahead with a certain response time and sensitivity to change.

These models indicate that a wide range of behaviour is possible, depending on the volume of traffic -- defined by the density of vehicles and their average speed. At low volume, cars move more or less independently, achieve their preferred speed, and change lanes at will. At high volume, the smallest disturbances trigger a jam of slow-moving or even stationary traffic.

At intermediate volumes, the flow may become 'provisionally stable'. Small local perturbations to the flow die away. Larger ones amplify into a jam. Congested regions can be pinned to the point of disturbance, for example extending back from an exit lane, or they can travel in waves back down the flow.

All of this is reasonably well understood, and may help traffic planners to design speed restrictions and junctions that minimize jams.

But Takashi Nagatani of Shizuoka University in Japan has identified a new hazard: the 'fluctuating vehicle'. He says that on a one-lane highway, a single car within a stream of traffic can send waves of congestion propagating down the line behind it simply by varying its speed, even if it maintains the same average speed as the rest of the flow.

This kind of driver is quite common. The parent distracted by children who then accelerates to catch up with the vehicles ahead; the tired lorry driver whose attention is wandering; the ageing car that struggles on small inclines but makes up for it downhill -- could all show greater speed variations than the rest of the traffic.

Nagatani's model indicates that the transitory increase in traffic density behind this car, as others slow down to compensate (or maybe just to keep a cautious distance), can grow into a persistent cluster of dense traffic that moves backwards relative to the fluctuating vehicle. And that it soon becomes unclear that the one caused the other.

The erratic car can trigger a whole train of these density waves, which move upstream like shock waves. The situation worsens as the average speed of the flow (which is the fluctuating car's average speed too) slows. First, the waves of congested traffic grow until the traffic is more like a fully congested flow with occasional regions of low-density traffic. Then, for still slower speeds, even these 'free flow' regions vanish, and a jam of congested traffic propagates backwards behind the fluctuating vehicle -- which, because its average speed is still the same, is probably oblivious to the chaos it is leaving in its wake.

So in medium traffic, says Nagatani, a single individual driving erratically enough can induce most of the jamming usually seen only in heavy traffic.