JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards
Section: Road Transportation Informatics

Probe Vehicle System Concept

Acquisition of road traffic data is an important aspect of Road traffic Informatic systems. An innovative approach is utilizing the vehicles themselves as a source of real-time traffic data, functioning as roving traffic probes. This principle was probably first touched upon in the early 1970s in the course of a pilot with the Japanese Comprehensive Automobile Traffic Control System (CACS), which was a government sponsored program aimed at the development of a route guidance and traffic information system. Besides inductive loops to transmit guidance information to CACS-equipped vehicles, also employment of the vehicles themselves to collect traffic data was propounded.

The development of present-day in-vehicle equipment started with

  1. autonomous (electronic) positioning and navigation devices, to assist drivers in the process of way finding and providing information on the current location and how this relates to the destination, and
  2. communication systems for gathering and dissemination of information (e.g. RDS)
A combination of static navigation systems and dynamic traffic information was the next step, and resulted in the first types of dynamic navigation or guidance systems.

Probe Vehicles: Floating Car Data

At a sufficiently high percentage of vehicles equipped with a dynamic route guidance (DRG) device acting as roving traffic detectors, a non-infrastructure based traffic monitoring system results. Furthermore, not only DRG-vehicles can collect experienced traffic data as it is certainly conceivable that certain (categories of) vehicles are specially equipped for performing this task (e.g. vehicles from the freeway service patrol or roadwatch).

Such vehicles, which are participating in the traffic flow and capable of determining experienced traffic conditions and transmitting these to a traffic center are called probe vehicles.

To determine its position and to register experienced traffic conditions, a probe vehicle is equipped with on-board electronics, such as a location and a communication device. By means of the location device, the probe vehicle keeps track of its own geographic position. By means of the communication device, the probe vehicle transmits its traffic experiences via a mobile communication link to a traffic center. In this traffic center the traffic data received from the probe vehicles is gathered, possibly combined with data from the other monitoring sources, and processed into relevant traffic information. In the traffic center decisions are made as whether to take corrective actions (e.g. ramp metering), supply information (e.g. variable message signing) or disseminate information directly to the car drivers (e.g. travel advisories, traffic information and dynamic route guidance).

Such ATIS System concept comprises the following components:

Traffic Center

The traffic center includes the physical building, equipment (hardware, internal computer networks), software and procedures, and persons within this building. In this way, they constitute the information systems that administer the traffic processes. According to the ultimate objective of the traffic center we discern a Traffic Management Center (TMC) for the ATMS purposes and a Traveler Information Center (TIC) for the ATIS purposes. The task of both the TMC and the TIC is to process the received probe messages, for the TMC the probe data is combined with data from other sources, into relevant information about actual and predicted traffic conditions.

Probe Vehicles

In the probe vehicle concept vehicles themselves are acting as roving traffic detectors, which are not bound to specific and fixed locations along the road infrastructure. Several strategies can be adopted for constructing the traffic messages to be transmitted by the probe vehicles. For instance, each probe can transmit a traffic messages once every time interval containing its location and its speed at the instant of transmission. The time interval will be shown to be either more or less fixed or to be exogenously determined. Determination of the instantaneous location of the probe can take place according to various techniques.

As travel times between two points are more reliable than measurements of the speed of vehicles at one particular point along the road, probe vehicles participating in the road traffic and automatically reporting the (link) time needed for traveling between two intersections are a precious source of road traffic data. Therefore, the probe reports should contain travel time information instead of spot speed information.

As the concept of traffic monitoring through probe vehicles has the potential advantage that also actual information about traffic flows in urban road networks can finally be obtained, the location technique to be chosen for the probe vehicle application has to have a high performance in urban areas. For this reason one could adopt a hybrid positioning system, combining differential GPS (DGPS) with map-matching and dead-reckoning. The reliability of this combined location method is very high and the accuracy is about 2 to 10 meters. In this way, a serious limitation remains that, even on freeways, differentiation per lane is impossible and hence all further computations for extracting traffic information from probe data are necessarily restricted to a conjoined freeway.

Mobile Communication Network

Conscientious transmission of the traffic data experienced by the probe vehicles to the traffic center (either TMC or TIC) is one of the most crucial aspects of probe vehicle traffic monitoring, because probe vehicle data will play a significant role in advanced traffic systems and communication characteristics may considerably affect the quality of this data source. Several options exist to send data from the probes to the infrastructure

Example

For the San Francisco Bay Area we worked out the performance of a probe vehicle system. The performance of ALOHA access is compared with polling with cellular frequency reuse.

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JPL's Wireless Communication Reference Website Marcel Westerman and 1993, 1995.