London's congestion charge is intended to force 20,000 commuters to switch to public transport. Credit: EPA/PA

The streets of London were once said to be paved with gold, but modern-day prospectors are more likely to find them filled with traffic. In a bid to lose London's tag as one of the most congested places in Europe, its local authority is about to start charging people who want to drive into the city centre.

Whether or not the plan succeeds, it will provide the most important test so far of the scientific models used to design such schemes, and transport researchers are itching to see what happens.

“London is essentially being turned into a big laboratory,” says David Milne, a traffic researcher at the University of Leeds. “Road pricing schemes have been kicked around in modelling terms in the United Kingdom since the 1960s, but we haven't any real evidence of how people respond.”

From 17 February, people wishing to drive into central London during the day will have to pay a £5 (US$8) fee. The move is intended to force an extra 20,000 daily commuters onto public transport. Cameras will register the number-plate of every vehicle entering a 13-square-kilometre central zone.

If the models prove accurate, city-centre congestion should fall by up to 30%, say those running the £200-million scheme. London is not the first city to introduce such a toll, but transport experts say that the scheme, which is run by Transport for London, is the largest and most significant of its kind.

Traffic flow in the models is estimated by considering the amount of road space, and how many people want to use that space. If the models have a weak spot, it is likely to be the latter estimate, say transport researchers. Commuters have been interviewed about how they will react to the charge, and the success of the scheme could rest on whether people behave as they said they would.

“Theory and reality can be miles apart,” warns a member of the working group that designed the scheme who did not want to be identified. Commuters will be swayed by the actions of those around them, for example, and emptier roads could even tempt bus and train users to get back behind the wheel instead.

Traffic flow around the edges of the charging zone is another area of uncertainty. Traffic levels in these areas could rise as drivers who don't want to pay the charge spill onto other routes. Milne says that traffic could increase as far away as the M25, the already-packed motorway that circles London some 30 kilometres out. But Transport for London points out that the working group's models predict that traffic in these areas will increase by 6% at most, and that levels there are lower anyway.

Transport for London has already announced a programme to monitor both of these areas, along with other effects of the congestion charge. The first results, based on follow-up interviews, vehicle-counting and measurements of average journey times, will be published in six months' time, although natural fluctuations in traffic levels mean that it may be years before solid conclusions can be drawn.

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