At a London gathering of more than 500 experts, advisers will urge the Department of Transport (DoT) to be explicit about toll enforcement, as it decides the technical features of road pricing. Trials on a few thousand cars are planned for the M4 or M25 in less than two years.
The sophisticated electronic technology needed to make road pricing work is proven, but only at prototype stage. Technologists want to know the Government's targets on enforcement so their designs can meet the challenge.
It should also ease public acceptance of tolls if people know beforehand how the authorities will compel the public to pay, according to Peter Hills, director of the Transport Operations Research Group at the University of Newcastle Upon Tyne.
Professor Hills is one of the DoT's key advisers, and a speaker at today's meeting. 'To have a system that accuses people of not paying when they have, or charges them twice because it is over vigilant, would be just as damaging as a system so lax it becomes widely known that cars can get away without paying,' he said yesterday.
Simple decisions such as whether systems must be sharp enough to pick up motorcycles hidden behind lorries could be critical in dictating the sensors and transmitters used.
Other basic questions remain unanswered. It is not yet clear whether failing to pay a road toll will be made a criminal offence. The problem is whether to distinguish between the old man who strays onto the M5 on a Sunday afternoon without the kit in his car to pay, and persistent violators. Professor Hills suggests an approach that is pragmatic, but has few supporters in the police establishment.
Most proposed toll systems store images of the licence numbers of cars that pass but do not pay. Professor Hills says these could be scanned to pick up cars that fail to pay perhaps five times in a week, thus ignoring one-off defaulters and concentrating the cost of enforcement on persistent offenders.
Early generations of road tolls were based on read-only tags that identified vehicles to a roadside computer. Then came systems whose tags could be altered, not just read. Beacons on gantries need not know the identity of each car, but use two-way communications with a 'smart card' in a unit inside the car to deduct payments. This makes roadside computers redundant, so avoids worries over privacy.
However, the Government has reasoned that offenders forfeit the right to privacy and it wants to be tough on enforcement. This means cameras sharp enough to spot and log non-payers within huge volumes of traffic.
The DoT has decided against the kind of 'toll plazas' used at the Dartford tunnel, for instance. But monitoring traffic on the move imposes huge technical demands. Equipment must cope with massive volumes of cars, and also vehicles travelling at high speed, possibly straddling lanes.
This requires a whole new approach. The most ambitious option uses satellites to broadcast signals that dock credit from in-car units. Local broadcasts, using cellular communications, might prove cheaper. Both approaches would require roadside cameras, because cars without toll units could avoid paying. Short wave radio links or infra-red communication are a possibility, although the favourite is microwave communication.