A highway stormwater runoff pollutant loading model has been developed based on results from composite sampling of approximately 600 storms at nine locations in the State of Washington over five years. The model expresses total suspended solids (TSS) loading in proportion to the product of highway segment length, average runoff coefficient, and vehicles traveling during storm periods. It was demonstrated that loadings of contaminants such as chemical oxygen demand, nutrients and trace metals could be estimated from TSS loadings using ratios derived from the data. The model described was developed and validated for assessing total loadings over a time span encompassing a number of storms (monthly or annually). To predict pollutant concentrations and loadings in runoff from a given storm, cumulative distributions were plotted and analyzed to determine the probability of exceeding specific concentration and loading values in a given case. Bioassay studies using highway runoff indicated toxicity volumes (in excess of 10,000 - 20,000 vehicles per day) or high metals concentrations in rainfall caused runoff concentrations to exceed lethal levels. Draining highway runoff through grass channels 60 meters in length greatly reduced TSS and metals concentrations and the consequent toxic effects. The major product of this research is a guide to assessing and mitigating the impacts of highway runoff to receiving waters.