But toll bridges present different challenges that must be approached from a different angle. They are relatively short by comparison to roads, with a large number of regular users, and the configuration of the structure rarely lends itself to construction of a toll plaza of suitable proportions. Often an area on land at one side of the crossing has to be set aside to provide sufficient capacity. What's more, they are usually the only available route, rather than being chosen by motorists for convenience.

Because the length of tolled highway is short, bridge operators have an obligation to try and minimise the amount of time motorists spend at the toll plaza. Not only will this make the journey less stressful for drivers, it will enable more paying customers to pass across the bridge, maximising revenue.

According to Philippe Monier, who is road business development director of toll equipment manufacturer Thales Transport Activities, the majority of bridge tolling systems in use at the moment are based on cash payment of tolls. This is not ideal, he says, because it requires a lot of staff, a large toll plaza, and mechanical equipment which needs frequent maintenance and is prone to breakdown.

Electronic tolling, which is already widely used on toll roads, is gradually being introduced on bridges, but Monier says that for bridges, the adoption of some kind of electronic system is an absolute must. "This system enables a free flow mode," he explains, which means that not only do motorists not have to stop, they don't even have to slow down or change lanes to pay their toll."

Of course such a system is only generally adopted by regular users, and bridge operators still have to accommodate occasional users, but one of the main benefits for bridge owners is that such systems are likely to be widely accepted, given that a high percentage of motorists will be regular users.

The ideal situation for bridge operators, explains Monier, would be to have a combination of electronic systems to address both needs. The first system, which is already used extensively on toll highways across the road, would allow regular users to sign up for an electronic tag to be carried in their car. To collect the toll, information is exchanged between a dedicated short range communications microwave or an infrared beacon which is installed above road level, and the tag or badge in the motorist's car. The badge contains all the information that the system requires in order to charge the user.

When the driver passes the first beacon, it checks that his subscriptions is valid, records the point of entry in the badge, where applicable, and opens the gate. At the exit, the system reads the information in the badge, including the point of entry, calculates the payment due, and transmits the payment data. Motorists may receive a monthly bill, or may pay by subscription. Two types of system are in use for operating electronic tolling - the single lane system, which is used in many European countries, and the free flow system, which is used in north and south America, Australia and Israel. The latter is technically more complex, but requires no gates - motorists do not have to slow down, or even change lanes, data is captured across the full width of the road.

As well as providing this sort of system for regular users, bridge operators would also have to consider how to collect tolls from occasional users without resorting to cash booths. The ideal solution, explains Monier, would be the same licence plate recognition system that is being used in London for the city's congestion charging programme. This is effectively a free flow system - no toll booths or gates are installed, instead information is gathered by video cameras positioned above the end of each street as it enters the charging zone.

The onus is on drivers to make the payment - they can either pay in advance, or at any time before the end of the day, without incurring a penalty. Motorists can pay by account, or occasional users can make payment online, by telephone or in certain shops where the facility is available. The operating system records the licence plate information and compares it to its database to ensure that payment is made.

Although this system is considered ideal for use on toll bridges, it is limited by legal restrictions in some countries, particularly in Europe. In these countries, for example in France, the law prohibits private companies such as toll operators from having access to vehicle licensing information.

This is likely to change in the near future, predicts Monier, especially in countries where local authorities and governments themselves have plans to impose new tolls such as congestion charging. Many motorway operators are lobbying for the laws to be changed to enable them to exploit this new technology to reduce congestion and improve efficiency.

The benefits to users are obvious - that they can pass toll gates without having to stop - but operators can also realise many benefits from these systems. Eliminating the need for staff makes the system easier to operate, particularly in poor weather, and improves safety. There is no mechanical equipment hence less moving parts requiring maintenance and susceptible to breakdowns. Toll operators can also collect money in advance with electronic systems, perhaps by offering incentives for motorists to take up monthly subscriptions, for example. The cost of a tolling system represents a small percentage of the overall cost of a bridge - usually around 1%. Monier says that the market price for a toll lane with cash and electronic tolling is about US$125,000.

Another issue for bridge operators to consider is interoperability, a buzzword which is already a reality in Europe. This means that the same in-car system can be used to pay tolls electronically for a number of different toll roads - even for different operators, or for roads in different countries. A pan-European road tolling scheme is the ultimate aim, but again, Monier concedes that this is limited by legal restrictions on the use of automatic numberplate recognition in some countries. The development of an effective enforcement system also has to be undertaken in order for such a scheme to be achievable. Such issues need to be resolved quickly, says Monier, before individual countries invest too heavily in their own solutions.

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