Transactions in GIS, 2005, 9(4): 585–601 © Blackwell Publishing Ltd. 2005. 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA. Research Article Improving Spatial Accuracy of Roadway Networks and Geocoded Addresses Jun Wu Department of Environmental Health Sciences University of California, Los Angeles Frederick W. Lurmann Sonoma Technology, Inc. Petaluma, California Tami H. Funk Sonoma Technology, Inc. Petaluma, California Arthur M. Winer Department of Environmental Health Sciences University of California, Los Angeles Abstract Exposure to traffic-related pollutants is associated with both morbidity and mortality. Because vehicle-exhaust are highly localized, within a few hundred meters of heavily traveled roadways, highly accurate spatial data are critical in studies concerned with exposure to vehicle emissions. We compared the positional accuracy of a widely used U.S. Geological Survey (USGS) roadway network containing traffic activity data versus a global positioning system (GPS)-validated road network without traffic information; developed a geographical information system (GIS)-based methodology for producing improved roadway data associated with traffic activities; evaluated errors from geocoding processes; and used the CALINE4 dispersion model to demonstrate potential exposure misclassifications due to inaccurate roadway data or incorrectly geocoded addresses. The GIS-based algorithm we developed was effective in transferring vehicle activity information from the less accurate USGS roadway network to a GPS-accurate road network, with a match rate exceeding 95%. Large discrepancies, up to hundreds of meters, were found between the two roadway networks, with the GPS-validated network having higher spatial accuracy. In addition, identifying and correcting errors associated with geocoding resulted in improved address matching. We demonstrated that discrepancies in roadway geometry and geocoding errors, can lead to serious exposure misclassifications, up to an order of magnitude in assigned pollutant concentrations. Address for correspondence: Arthur Winer, Environmental Health Sciences Department, Room CHS 46-081, School of Public Health, University of California, Los Angeles, CA 90095-1772, USA. E-mail: amwiner@ucla.edu.