A new approach to traffic data collection, known as link monitoring systems-systems that "mine" location data from existing infrastructure (such as cellular telephone networks or fleet management systems) to estimate link speeds and travel times-is emerging. In general, these systems are being developed and operated by private firms, which then seek to sell the data to transportation agencies. To make sound purchasing decisions, agencies must evaluate the quality of the data that they are considering purchasing. The research effort described in this paper developed and field-tested a link monitoring system evaluation procedure. This procedure is based on the use of state-of-the-art information technology resources, namely, geographic information systems and Global Positioning Systems (GPS), to provide a flexible, thorough, and fair evaluation methodology. The methodology consists of four steps. Step 1 is baseline data collection, which includes study site investigation, route design, and probe vehicle runs. Step 2 is data reduction, involving GPS data postprocessing, spatial processing, and travel time and link speed derivation. Step 3 is temporal match, and Step 4 deals with comparisons, including accuracy quantification and temporal traffic condition comparisons. The paper illustrates the procedure based on an evaluation of the Airsage link monitoring system in Hampton Roads, Virginia. In this evaluation, 147,000 GPS points that supported comparisons with over 1,000 link and time data records provided by the Airsage system were collected.