The Challenges of a True Distance Approach to Road Charging

The Challenges of a True Distance Approach to Road Charging


D G Matheson, PA Consulting Group, UK


This paper investigates the implications of the challenge to design an OBU that can levy charges on the basis of distance, time and place questions whether it can be met and at what ?cost?.



Traffic congestion in the United Kingdom represents a major constraint on economic efficiency, which all forecasts predict will continue to worsen in the years to come. The UK has a limited range of options to address this concern. Congestion charging is one potential option, but to implement this effectively, all vehicles would need to be equipped.

As part of its ongoing programme of transport improvement, the UK Secretary of State for Transport set up a Committee to investigate the feasibility of options for road charging in the UK as a potential approach to addressing this increasing problem. In July 2004, the Road Pricing Feasibility Study Report was published . The report indicated that ?the key to a fully national road pricing scheme is a technology which can charge by time, distance and place to target the costs, including environmental costs.?


The requirements to charge in the manner envisaged will make particular demands on the capabilities of equipment installed in vehicles, the back office processes to manage the data collected, and for policy maker to ensure that the revenues are directed into areas that the public believes are for the greater good.
For some, the technology may be seen as existing today, particularly with the advent of the German Lorry Charging scheme introduced in January 2005. However, closer inspection of the capabilities reveals that the distance charging being performed:
· Only applies on the German motorway network
· Comprises predetermined charges related to the length of each motorway link (to give a consistent result) rather than true distance measurement; so-called segment charging
· Requires the support of additional (infra-red) beacons to assist the OBU in differentiating between certain stretches of motorway and parallel roads.
and hence is limited in location terms and does not address the time of day component ?required? by the UK study.

Alternatively, the successful scheme implemented in Switzerland in 2001 uses complex OBUs to measure the distance travelled within the country, using the tachograph as the primary measurement device, but not in a manner that addresses the location of the journey (except within the Swiss borders) nor the time of day.

Since both of these units represent some of the most advanced devices available in modest volumes today, and typically costing in the order of $500, it is clear that a device that could differentiate by time place and distance would need to be even more complex, potentially much more costly at today?s prices, and more difficult to engineer into a reliable and robust device. Implementation within a vehicle population of 30m, even ignoring the volume manufacture, roll-out and installation issues would represent a major investment, and for some, the only practical way forward would be for equipment to be fitted at the point of manufacture. This could be complement by other mechanisms to address the operating costs of older, unequipped vehicles.

Even allowing for these complexities, what might an ?all-encompassing? distance, time and location charging OBU consist of?

The ?Requirement?

For a car-based distance charging OBU there a number of reasonable requirements to be satisfied:

· A robust distance measuring capability
· Differentiate distance travelled by road type or at the least, by zone and specific roads
· Support user privacy
· Communications on/off board should be ?cost effective? and not prone to user intervention
· Any need for mapping data should be ?up to date? and cost effectively handled either on or off the vehicle
· Tariff table should be ?relatively? straightforward to define and store / implement
· Cheap both to supply and to install

Solution Options

Whilst lorries in Europe generally have to be fitted with a Tachograph for safety reasons (driver hours), they are type approved devices with known tolerances (+/-4%) on the ability to compute distance travelled. The also have security seals to prevent tampering and are subject to regular inspections by government agencies. In contrast, motor cars have odometers that are not robust and tamper proof and cannot thus be relied upon to deliver a true estimate of distance for charging purposes. They are also less accurate and are not type approved.

As with all measurement techniques, there is always some degree of error, and for GPS based position determination this could typically be in the order of a few metres.

The majority of GPS receivers available today typically provide position updates at once per second during which time the vehicle could have moved some distance away from the true location at that time. Appropriate processing can overcome this to a large degree, given the relatively modest vehicle performance even under heavy acceleration or braking. Some new GPS receivers on the market have the capability to deliver position updates at ten times per second which could offer a considerable improvement.

As indicated, segment charging is the technique adopted for the German lorry charge. This typically involves capturing a number of position fixes along the motorway section and comparing these with an electronic map of the road partitioned into sections. If critical sections along the link can be matched to the position estimates obtained by the OBU, then the unit can be ?sure? that it is on the motorway and can levy the charge appropriate to that link.

Moving to the more complex environment of an urban location makes this much more difficult, partly due to signal propagation effects, but also because the road network is much more complex and the need to define the roads into segments for comparison with position estimates becomes trickier. This would introduce ambiguity into the route undertaken, and hence the distance travelled.

The paper will go on to discuss the likely components and pre-requisites necessary to provide an OBU that can be used as a basis for true distance charging, the challenges that these impose, and whether such a device is a practical proposition.


Association for European Transport