ABSTRACT:
This paper presents a new charging scheme based on distance vehicle traveled in different areas determined by GPS positioning and digital road network database in GIS. The system architecture is designed and system modules are presented. The new system not only more consistent with road pricing principles and objectives of reducing traffic congestion and air pollution, but also more flexible for the integration of ERP system with other Intelligent Transportation Systems (ITS). The capital investment and operating costs are less than traditional ones and is relatively simple to implement and modifying.

1 Background
Road pricing has been effective in managing congestion on roads in the metropolis of Asia and all over the world. Changes have been made to the road-pricing scheme since its implement from a manual scheme based on paper permits to an electronic version. Technologies like Global Positioning System (GPS) and Geographical Information System (GIS) make the expansion and more effective of the road pricing scheme possible.

This paper presents a new charging scheme for the next phase of Electronic Road Pricing (ERP) system based on GPS and GIS. The system architecture is designed and some basic modules are discussed. The new scheme charging will be based on distance vehicle traveled in different areas determined by GPS positioning and digital road network database in GIS. This makes the new system not only more consistent with road pricing principles and objectives of reducing traffic congestion and air pollution, but also more flexible for the integration of ERP system with other Intelligent Transportation Systems (ITS) such as emergency assistant and dynamic traffic assignment. At the same time, it has lower capital investment and operating costs, and is relatively simple to implement and modifying.

2 System Design
The new designed system consists of the management center, the in-vehicle unit, and the communication link, and some program modules.

2.1 The management center
The proposed management center of next ERP system is functioned to tracking, monitoring, charging and guiding all the vehicles traveling on the road. Charging and guiding scheme is on the basis of collected traffic information and map database. Figure 1 shows the architecture of management center.

Communication server is used to exchange information through Internet, which is the interface between the in-vehicle unit transceiver and the management center. It is not only used for vehicular position to be sent to the management center from the vehicle side, but also for the road map to be transmitted to the vehicle when it requires to display but has no map itself. The guidance command and charging scheme are also transmitted to the vehicle from the communication server. Traffic information analysis provides the statistics of traffic flow on the road on the basis of the real time vehicle position, makes charging scheme for each road and guiding scheme for the vehicle. Tracking and Monitoring provides an interface for the operator in the management center.


Fig. 1 The diagram of the next ERP system management Center



2.2 In-vehicle unit
The proposed in-vehicle unit in next ERP system is shown as in figure 2. Integrated GPS/Dead Reckoning (DR) positioning provide continuous vehicular position output even in urban area where dense high buildings or the tunnel will block the GPS signal. To further improve the positioning accuracy, Map Matching (MM) could be applied in the in-vehicle unit if there is map database and enough processing power or implemented in the management center. The accurate positioning information could be used to assist the traffic flow analysis on the exact road. The Data fusion is mainly for fusing all the sensors' data to get more accurate, robust positioning information. Status provides the vehicular or road status such as alarm, breaking down or congestion. Display shows some related information to the driver including guidance, charging information or electronic map if there has. Transceiver provides two-way data link via wireless network and IP based packet data transmission. The data would be broadcast into the Internet so that any authorized user can make full use of them to provide value-added services. Control card is the crucial part for organizing all these information. Smart card interface is for charging via smart card inserted into the in-vehicle unit.


Fig. 2 Design of the in-vehicle unit for ERP

2.3 Communication link
Wireless communication is used to transmit all the vehicles position and status to management center, and broadcast the updates of the traffic information when required or periodically.

For Singapore next ERP system, current cellular mobile systems or third generation wireless communication could be employed and the mobile set could be embedded in the IU. As 2.5 Generation wireless communication, General Packet Radio Service (GPRS) has quicker session setup, permanent connection, lower cost, higher data rate performance[5]. Furthermore, it is IP based data transmission so the transceiver is unnecessary in the management center, which is connected with Internet. It is better choice for the next ERP system and can be easily upgraded into the next generation wireless communication.

3 System Modules
Based on the proposed architecture of in-vehicle unit and management center, the main modules include electronic map database, integrated positioning, and map matching.

3.1 Electronic map database
In next ERP system in Singapore, electronic road map could be used for assist positioning, charging scheme and route guidance. The contents and data structure in the electronic map database are described as follows.

1) Spatial database: This includes node, which may be the junction and cross point of road, link representing road segment. The points between two adjacent nodes are also included in the spatial database.

2) Non-spatial database: This includes address name, road name and attribute etc.

3) Topological relationship: It includes relationship between node and node, node and link, among links.

3.2 Positioning of vehicle
In next ERP system, widespread installation of expensive gantries is not necessary because the position of vehicle will be used for charging. There are many positioning systems are available for the civil application; among them GPS could provide 10 meters accuracy without SA and can work in all weather, at any time of the day, and under specified conditions of radio-frequency interference, signal availability. However, in an urban environment, satellite signals are blocked by high buildings or heavy foliage, and the urban canyon limits satellite visibility. To solve these problems, many aided positioning methods such as dead reckoning (DR), wireless communication were adopted [3]. In comparison to DR, wireless aided method is still dependant on the radio environment. DR uses the distance and direction changes to provide the vehicular position relative to the original point. As DR has error accumulated over time and reliable GPS solution are often not available in urban area, multi-sensor integration systems, therefore, are required for next ERP systems, which is shown in the diagram of figure 1. The GPS receiver is for the absolute positioning and set up the original point for DR, while the distance and direction sensor provide the distance and direction information for DR algorithm. When the GPS data is available, the fusion algorithm predicts the errors of distance and direction sensor; the DR algorithm will provide the positioning while GPS data is not available.

The traffic flow statistics will be dependant on the road where vehicle travels while its position is transmitted to management center. However, sometimes the GPS and/or DR cannot give an accurate position, which cannot determine which road the vehicle travels on. Therefore map matching is needed to integrate with GPS/DR in next ERP system. Map matching is simply a method of using stored electronic map data information about a region to improve the ability of a position determination system to handle errors[4]. Its algorithm has basically two categories: as a search problem or as a statistic estimation problem. As a search problem, map matching is to match the measured vehicle position to the closest node or point within a predefined range in the road network. This approach is fast and fairly easy to implement and it is suitable for the vehicles that have fixed track such as buses or travel in accordance with planning route. As a statistics estimation problem, map matching use probabilistic approaches to make the match more accurate through modeling errors in an ellipse or rectangle of uncertainty that represents the area in which the vehicle is likely to be located. It is suitable for any vehicle that does not have fixed route in traveling.

3.3 Map matching
Map matching algorithm in CCS processes the received vehicle position and searches the map database to create the list of road links the vehicle traveled. The required payment is calculated according to the road link price in database and deduction is made from prepaid card or stored to the vehicle ID account for monthly-based payment.

Different from vehicle navigation system[6], map matching for ERP does not require real time calculation. This means that the searching of road link vehicle traveling on current can use not only the past and present vehicle position but also the following vehicle position. The off time matching makes it much more accurate and reliable. The matching processes (as shown in figure 3) are: initialization (O), position between intersections (M-N), position near intersection and no turn detected (M), position near intersection and turn detected (P), and re-initialization.


Fig. 3. Map matching process



A probabilistic algorithm is designed to match the initial position of the vehicle. Since the actual location of the vehicle is never precisely known, so we determine an error ellipse, i.e. confidence region, that vehicle is likely to be within. From estimation theory, the input and output signals can be modeled as stochastic process. Variable associated with the true and measured values can be modeled as random variables. Variance-covariance information is propagated through appropriate algorithms to derive the variances and co-variances as functions of the original random variables or as functions of parameters estimated from the original observations. These variances and covariances are used to define confidence region. The determination of the confidence region should also consider the map accuracy as well as the road width.

Searching process proceed until there are candidates within the region. A match completed if there is only one road link cross or within the region. If more than one candidates exist, the candidates are eliminated with the following standards until the only correct link is matched: direction difference between road link and vehicle traveling, traffic restrictions such as one-way road, distances between vehicle position and candidate link. One matched link can be used to verify the immediate past link with their topology relationship.

After a valid start point is known, only three situations are to be considered: vehicle on a road link between intersections, vehicle near an intersection while no turn is detected, and vehicle near an intersection while a turn detected.

Suppose the route traveled is O-M-N-P-Q, the map matching processes are as following (fig. 3).

  • Matching the start position to initial location O on link L1, record the distance OM by GPS and DR distance sensor.
  • When vehicle near node M, three possible connections are considered and match to link L2 while no turn is detected at M.
  • Among 3 possible connections at N, when a turn is detected, link L3 is selected according to azimuth measurements or angle turned.
  • Repeat the same processes above.
  • The vehicle trace recorded is L1-L2-L3-L4, …
Suppose the recorded links in one month for a vehicle are Li (i=1,2,3, … , m), then the monthly payment P for the vehicle can be calculated according to price scheme in the database,



4 Conclusions
When vehicular position is collected via wireless communication, traffic flow analysis will be made in the management center to provide dynamic route guidance for vehicle on the road and then improve the quality of service of the transportation. If MM is done in the management center, no additional cost is added into the in-vehicle unit. When MM is done in the in-vehicle unit, additional storage space is required for map database, but the storage media is so cheap even for hundreds of megabytes memory that it can be neglected. But How to make the charging scheme and accurately analysis the traffic flow to provide efficient dynamic route guidance need to be further studied.