Advances in alternative fuel technologies and increasing motor vehicle fuel efficiency may result in a decline or flattening of demand for conventional fuels in the United States in the coming decades. One outcome of this development may be a reduction in state and federal income from motor fuel taxes, the primary source of funding to maintain and improve the transportation infrastructure. To keep pace with future transportation needs, a new funding mechanism is needed to supplement or replace the current road financing mechanism. One possible approach is to charge for road use based directly on a measure of travel on public roadways by using onboard computers coupled with the Global Positioning System (GPS) and digital maps. The main goal of this research was to develop the system requirements for the GPS component that would determine the vehicle's location on a given roadway with an in-vehicle road user charging system. The focus was to evaluate the GPS in the most difficult of environments - locations in which roads of different jurisdictions and possibly different fee structures are located in proximity to each other. To be effective, the system must be able to place the vehicle on the correct road. A description is given of a methodology to specify the needed accuracy of a GPS receiver to meet a required accuracy in distinguishing roads with a given separation distance at a statistical level of confidence. Results are reported for a series of road-based experiments that were performed to evaluate GPS receivers that use the nationwide differential GPS service.