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86 I N T E R N AT I O N A L P E R S P E C T I V E S O N R O A D P R I C I N G to be available in 2008. Since the satellite navigation sys- in which charges are levied on all travel throughout a tems provide only one-way communication (from the defined area on the basis of distance traveled, time spent satellites to the receivers), a cellular phone system is nor- traveling, or perhaps time spent in congestion. mally used for communication between the vehicle and A recent survey of the policies adopted by U.K. city the control system for the transaction process. For planners indicated that they typically adopted a simple GPRS, the position can be determined by the data con- approach by focusing on the city center and any major nection via the mobile phone network with an always-on traffic generators on its fringes. The single cordon connection. The resulting VPS allows a more complex would be placed just inside the inner ring road around charging regime to be implemented. The system also the center, with crossing points minimized where possi- requires minimum infrastructure on the roads. The fee ble, a uniform charge to cross at all points, and that can be defined on the basis of crossing points, presence charge kept low enough to be publicly acceptable in an area, or per unit of time or distance. The fee struc- (Sumalee 2001). ture could be stored either at the main control center or Conversely, theory tells us that the "first-best" charg- in the IVU. A similar system was tested in Hong Kong ing regime is one that results in drivers on each link in and was proved to be reliable (Catling 2000). The Swiss the road network incurring the marginal cost of travel and German governments also launched the first large- on that link (Sheffi 1985). Such charges are impractica- scale GPS-based project, which will soon be operable ble and could prove expensive to implement, but they and will charge HGVs on the basis of distance traveled serve as a benchmark for assessing real schemes. The (Guillermo Jordan et al. 2001). The key barrier to large- question of where best to locate a single cordon or a scale implementation, especially in an urban area, is the given number of charging points is altogether more chal- required level of accuracy of the positioning system. At lenging and has been addressed by relatively few the moment, the accuracy of the GPS system is 10 to 15 researchers (Hearn and Ramana 1998; Shepherd and meters, whereas Galileo promises to deliver positioning Sumalee 2004; Verhoef 2002). One important theoreti- accuracy down to 4 meters. Despite the potential cal study comparing parking charges, cordon charges improved accuracy, there are various blind spots in the around centers, continuous charges, and charges limited road network (e.g., tunnels) where the GNSS may expe- to selected lanes with free parallel routes suggests that rience problems. This can be overcome by integrating the last of these, which largely reflects HOT lane prac- the GNSS with the short-range communication system tice, is by far the least efficient in its impacts on network (e.g., communication with beacons) or a dead-reckoning performance (Small and Yan 2001). system (Ochieng 2003). A recent research project has used genetic algorithms to determine optimal locations and charge levels for dif- ferent patterns of charging points. In an application to Scheme Design and Integrated Strategies Edinburgh, it compared four single cordons largely based on planners' designs, the same cordons with varying The majority of proposals for road pricing have assumed charges, an optimally located cordon, and charging lim- that charges would be imposed to cross cordons or pass ited to 10 isolated points in the network. The planners' points in the road network. Point-based or cordon charg- cordons varied substantially in their performance, with ing is a remarkably flexible technique. It can involve single the best producing more than twice the benefits of the or multiple cordons, screenlines to control orbital move- worst. The optimally located cordon was about 25% ments, and point charges at particularly congested loca- more effective than the best of those suggested by the tions, with charges varying by location, direction, and planners. Relaxing the requirement for a closed cordon time of day. The early proposals for Hong Kong (Dawson and limiting charges to 10 key points added a further and Brown 1985), those studied in London (Richards et 20% to the benefits, and relaxing the need for uniform al. 1996), and the current scheme in Singapore (Menon charges at all points a further 20% to 60%. Charging at 2000) all exhibit this flexibility. 10 points, with variable charges, proved to be twice as However, even such complex charging structures effective as the best planners' cordon with uniform have been criticized. It has been argued that they are charges (May et al. 2002). While this research raises sev- inflexible since fixed charging points cannot readily be eral other questions, it suggests that there is much to be relocated, that they are inequitable because they impose gained by a more analytical and flexible approach to the the same charge for short and long journeys, and that location of charging points in urban networks. they are disruptive because they encourage rerouting to Shepherd (2003) investigated the relative perfor- avoid the charge. All of these limitations arise from the mance of cordons, a fuel tax, and a smart cardbased discontinuities that point-based charging introduces approach for Edinburgh within integrated strategies by into the road network. These arguments have led to the using the strategic model START. He concluded that alternative suggestion of continuous charging schemes, small city center cordons can create boundary effects