At the request of the Washington State Department of Highways a study was conducted to evaluate three computer models specifically designed to predict carbon monoxide (CO) concentration at receptor points along roadways. The models were evaluated on their ability to predict values obtained from a monitoring network established along various roadways in the State of Washington. Monitoring consisted of 12 ground level CO stations, 2 meteorological stations and traffic counters. Traffic speeds were sampled for conformity during peak hours. The final selected models, CALINE II and EPA's HIWAY, were given calibration factors to be used when calculating Receptor Concentrations for Impact Assessments.