For whom the road tolls

The UK government will be keeping a close eye on the ambitious national motorway road pricing scheme being finalised in the Netherlands. Approved last year (2008), the system should start charging commercial trucks for road usage by 2011, and expand to every vehicle by 2018.

It will be based on a global navigation satellite system, (GNSS) anchored either on GPS or on the European Galileo satellite, which will track each vehicle's progress on a per-kilometre basis so that road usage can be charged for according to the distance travelled. The on-board equipment (OBE), required by law in all but officially exempt vehicles, will record kilometres driven as well as where and when they were travelled.

Recording the time of the journey is crucial because it will help the Dutch authorities charge according to the time of day the road was used, particularly peak hours when the per-kilometre charge is higher. A similar scheme is already running to track HGV traffic on German motorways, with another planned for France.

Dr David Linsdall is deputy technical director at communications giant Thales UK's research and development facility in Reading. He says using satellites to track vehicle progress has a number of key advantages over other road charging schemes which use fixed point sensors on bridges, gantries and virtual charging points, such as the central London congestion charge.

"GNSS offers the attraction of more centralised systems that do all the number crunching - you move the technology from the roadside into the vehicle, which makes it far more flexible," Linsall explains. "If you want to change the area you charge for, rather than putting up new roadside infrastructure, you just put in new map co-ordinates to build a new area of coverage."

The Dutch scheme will use a 'thin' - as opposed to a 'thick' - client OBE that transmits the vehicle's base co-ordinates back to a central office where computer systems will perform all the necessary time, distance and billing calculations. "The thin client does not need to store maps or tariffs, so the unit is cheap and has a lower overall cost for the user, but there is a privacy problem [because route details are transmitted to and held on central computers]," adds Lindsall.

Tracking road usage with satellite communications is not 100 per cent accurate, either. A research report by the UK's Transport Research Laboratory indicates that current satellite technology is only 99 per cent accurate when used for this type of application - a seemingly small margin of error, but potentially a huge problem considering the millions of vehicles the Dutch authorities expect to track. Monitoring travel on open roads and motorways is much easier than tracking journeys through town centres where there are high rise buildings and the in-car antenna can lose its view of the sky.

"If you are in a cordon scheme like London, it doesn't matter, but if you are charging for using a motorway that goes through a city, where there are parallel roads next to it, and even underneath it, vehicle routes might be misidentified and drivers mischarged," says Lindsall.

Recording the times of people's travel also raises questions about potential privacy infringement. The Dutch authorities have pledged to transmit only 'aggregate data' to the back office, and only to disclose detailed individual route and schedule information with the consent of the user. Each OBE will have a trusted element that ensures reports are digitally signed and encrypted prior to transmission.

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