Despite the doubts, and despite complaints from shop owners, London's congestion charge—introduced in February 2003—has managed to ease the gridlock in the city centre. Traffic is down by 18%, jams by 30%. The scheme's biggest weakness is that it is crude: drivers pay ￡8 ($14) to enter the zone between 7am and 6:30pm, regardless of how congested the roads are, or how long they stay. So road-pricing fans are watching trials by Transport for London (TfL) of a new detection system, called tag-and-beacon, with interest. Under such a scheme (used in Singapore and on some European roads) cars are fitted with electronic tags that are read by roadside masts. If the trial is successful, TfL says that the city could switch to the system once the contract to run the congestion charge is re-let in 2009. Currently, cameras are used to read license plates and track motorists. They are not always reliable: an individual camera identifies only around 70% of cars. Most driven get photographed more than once, which boosts the system's effectiveness to over 95%, but that still leaves several thousand vehicles per day whose details must be laboriously checked by hand. Tag-and-beacon technology is much more accurate, with an identification rate of over 99%. TfL says the trial is partly designed to see whether the new system could allow drivers to pay charges by direct debit. That would be popular with motorists, who complain that the current payment system is unfriendly: the toll for a day's travel must be paid manually—online, by phone or in a shop—by midnight, with steep fines levied on forgetful drivers. More precise detection also allows for more precision in policy, and road-pricing enthusiasts see radical possibilities ahead. TfL says it is considering using the new technology to charge drivers each time they cross the zone boundary (up to a daily maximum), instead of paying once for an entire day's travel. That would be cheaper for drivers who make few trips into the zone, although drivers who spend a long time trundling around without leaving (thereby causing the most congestion) would get off lightly, too. Further refinements may be possible. The current system has cut traffic most drastically in the middle of the day, when congestion is at its lowest. Demand for road space would better match supply if charges were variable—high at the busiest times of day and low in quiet periods. Such a time-sensitive, variable-charging scheme using a tag-and-beacon system was endorsed last year by Bob Kiley, the TfL's boss, who also said that he wanted to extend the congestion charge to other parts of London. That would be controversial, and Mr. Kiley's underlings were quick to insist that his musings were not official policy. But the original scheme was controversial too, yet Ken Livingstone, London's mayor and its biggest backer, was re-elected after introducing it. It would be a shame if timidity took hold now. From paragraph 1, we know that London's congestion charge scheme