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Winter 2003

Assessing road user charges using GIS and GPS technologies

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Shashi Shekhar (right)

Hybrid electric vehicles are already entering the market, with fuel cell engines not far behind. These and other advances in vehicle engineering offer solutions to the problems associated with excessive fossil fuel consumption. But they create a new challenge for state transportation departments throughout the country: as fuel consumption decreases, the revenue stream from motor vehicle fuel taxes – the main source of funding for state and federal highways – will eventually dry up.

Researchers at the ITS Institute are currently at the center of an effort to evaluate and refine ITS technologies that will enable real-time assessment of road user charges based on actual road use, including road type and jurisdiction, with other assessment parameters to be added in the future. The key components involve Global Positioning Systems (GPS) and Geographic Information Systems (GIS) based on computerized maps.

ITS Institute Director Max Donath leads the multidisciplinary team, which also includes computer science professor Shashi Shekhar, who is evaluating the GIS component, and Pi-Ming Cheng, research associate in the mechanical engineering department, who is evaluating the GPS component.

Because the system will be used to assess charges to motor vehicle owners, accuracy is of the utmost importance. The system must be accurate enough to discriminate between a vehicle traveling on a highway and one traveling on an adjacent frontage road. It must also maintain this accuracy in different types of terrain and in urban as well as rural areas, and be available nationwide.

The overall accuracy of the system is dependent on the accuracy of both the GIS and the GPS components; inaccuracies in either one can compound overall system errors. In the GIS component, positional accuracy may vary due to errors or uncertainties in data gathering.

In the GPS realm, achieving the accuracy required for reliable vehicle tracking (roughly one meter) requires a differentially corrected GPS system. The differential signal, broadcast from a base station on the ground, corrects for inaccuracies produced by the signal passing through the earth's atmosphere. In this application, the differential signal is broadcast from one of several Nationwide Differential GPS Service (NDGPS) base stations. A new base station in Pine River, Minnesota, recently joined stations in Wisconsin Point and Alma, Wisconsin.

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Measuring the accuracy of digital maps using high-accuracy GPS

The team obtained three competitive models of differential GPS receivers from different manufacturers as well as a reference system to be used for comparison purposes. The "gold standard" is based on a dual-frequency differential GPS, boasting one- to two-centimeter accuracy. Dual frequency differential GPS offers the ability to correct errors in the GPS signal caused by signal refraction in the ionosphere and troposphere. Despite their accuracy, the high cost of these systems makes them inappropriate for consumer vehicle applications.

Shekhar developed an experimental method to measure the positional accuracy of digital maps, by comparing digital road maps with measurements made using the "gold-standard" high-accuracy differential GPS system. In essence, the procedure begins by overlaying a digital road map with the GPS track for that road segment. A buffer zone is plotted around the GPS track, and the accuracy of the map is determined by finding how much of the mapped road falls within the buffer zone.

For a task such as differentiating between frontage roads and freeways, the researchers determined that acceptable map accuracy would be over 90 percent of the mapped road segments falling within a 40-foot buffer. Most maps do not meet this standard, achieving 90 percent accuracy only with much wider buffers--which implies that standard digital maps lack the needed accuracy.

This project breaks new ground in GPS research, says Donath. "No one has previously evaluated the accuracy of such differentially correct signals for dynamic tracking of vehicles, especially in an application where lateral accuracy is so important."

The research team has developed ways to improve the positional accuracy of digital road maps and is exploring their use in a road user charging system. In the future stages of this research, the team will begin to evaluate the maps' attribute accuracy, i.e., how well the maps are able to distinguish different road types and jurisdictions.

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