Investigation of Vehicles as Probes Using Global Positioning System and Cellular Phone Tracking

Problem

This study investigates the feasibility of vehicle tracking technologies for vehicles as probes (VAP). Two technologies were evaluated: Global Positioning Systems (GPS) and cellular phone tracking technologies developed by US Wireless.

The study consisted of three phases of project tasks: Phase I-program development and initial field test, Phase II-field operational tests, and Phase III-a large-scale field implementation. The overall goal of the study is to set up a VAP system.

Vehicle probe information can be complementary to those freeways that have single loop detectors because the single loop detectors are not capable of producing the most accurate traffic speed and density measurements. Probe vehicle data can also be used for rural freeways or minor urban freeways where no instrumentation is planned. Local streets and arterials are also good candidates for collecting data from probe vehicles.

Currently, no off-the-shelf software exists for VAP applications. A custom software package was developed as part of this project in order to conduct the technology evaluation.

Findings

Analysis showed that accurate location technologies are capable of producing travel time information for nearly all roads. A technology with 20-meter accuracy can produce data for approximately 99% of the road and freeway segments in the two counties (Alameda and Contra Costa in the San Francisco Bay Area) studied. Currently used GPS systems provide 15-meter or better accuracy. However, current cellular technologies typically provide 100-meter accuracy, with 50-meter accuracy mandated by 2002 (Emergency 911 system requirement).

Aside from location accuracy, results from the field tests show that variability in the positional accuracy and the total length of the tracking sequence are also important factors in the success of a tracking technology for VAP. With a sensing accuracy of 10 meters such as provided by some GPS systems, tracking the vehicle for twice the accuracy dimension (in this case, 20 meters of travel) is often sufficient to establish a travel time for the probe. However, for an accuracy of 100 m, such as provided by many current cellular phone technologies, the vehicle may need to be tracked for a mile or more to establish a travel time.

Other challenges of VAP, particularly for cellular phone-based tracking, include being able to filter out "uncharacteristic behaviors" from tracked vehicles, such as a vehicle stopped out of traffic by the side of the road, or a vehicle being towed, which might yield misleading data. Post-processing of the VAP data may be able to address some of these types of challenges.

Other challenges encountered with the technologies evaluated included being able to distinguish freeway travel from frontage road travel. All technologies also produced outlying data points that made tracking difficult.

Overall, GPS systems provided greater accuracy than cellular phone-based systems. Of the cellular technologies evaluated, at least one implementation shows promise, but needs further study to produce quantifiable travel time data.