Module 2 : Traffic Measurement Procedures
Lecture 08 : Automated Traffic Measurement
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ITS Probe Vehicle Techniques

Travel times using ITS components and passive probe vehicles in the traffic stream equipped with signpost-based transponders, automatic vehicle identification (AVI) transponders, ground-based radio navigation, cellular phones, or GPS receivers.

Some vehicles are equipped with dynamic route guidance (DRG) device which act as roving traffic detectors, a non-infrastructure based traffic monitoring system. 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.

Through the communication device, the probe vehicle transmits its traffic experiences via a mobile communication link to a traffic center. For instance, each probe can transmit traffic messages once every time interval containing its location and its speed at the instant of transmission. In this traffic center the traffic data received from probe vehicles is gathered, and combined with data from the other monitoring sources, and processed into relevant traffic information. It is very useful for Advanced Traveler Information system (ATIS).

Advantages
  1. Low cost per unit of data
  2. Continuous data collection
  3. Automated data collection
  4. Data are in electronic format
  5. No disruption of traffic
Disadvantages
  1. High implementation cost
  2. Fixed infrastructure constraints - Coverage area, including locations of antenna
  3. Requires skilled software
  4. Not recommended for small scale data collection efforts
ITS probe vehicle data collection systems
  1. Signpost-Based Automatic Vehicle Location (AVL) - This technique has mostly been used by transit agencies. Probe vehicles communicate with transmitters mounted on existing signpost structures shown in Fig. 1.
    Figure 1: Signpost-Based AVL Communication Processes, Source: Travel Time Detection Hand Book
    \begin{figure} \centerline{\epsfig{file=qfAutoSignpostBasedAVLComm.eps,width=8cm}} \end{figure}
  2. Automatic Vehicle Identification (AVI) - Probe vehicles are equipped with electronic tags. These tags communicate with roadside transceivers to identify unique vehicles shown in Fig. 2 and collect travel times between transceivers.
    Figure 2: AVI Vehicle-to-Roadside Communication Process, Source: Travel Time Detection Hand Book
    \begin{figure} \centerline{\epsfig{file=qfAutoAviComm.eps,width=8cm}} \end{figure}
  3. Ground-Based Radio Navigation - It is used for transit or commercial fleet management, this system is similar to the global positioning system (GPS). Data are collected by communication between probe vehicles and a radio tower infrastructure as shown in Fig. 3.
    Figure 3: Ground-Based Radio Navigation Communication Process, Source: Travel Time Detection Hand Book
    \begin{figure} \centerline{\epsfig{file=qfAutoRadioNavigation.eps,width=8cm}} \end{figure}