David Eves of the UK's Transport Research Laboratory says variables, such as weather and driver behaviour, make creating useful scientific models difficult. The more you think about the problem, the more difficult it becomes to predict what will happen on real roads. The idea behind car pooling is straightforward enough. Move more people from A to B, not more vehicles. The drawback is equally straightforward, says Mr Eves.
People are loath to share cars.
The temptation of all that empty tarmac has proved too much for some lone commuters. Tales abound of drivers resorting to using inflatable dolls and even corpses to fool the authorities. Mr Eves says the idea is falling out of favour and at best can only ease traffic flow at certain bottlenecks. Drivers in the United States are said to accept the fines for abusing HOV lanes as the price of getting to work on time. Given this, some of the lanes are being thrown open to those willing to part with a toll - which rises at busy times.