A 5-year effort to characterize highway runoff in Washington State resulted in the accumulation of data from more than 500 storms at nine locations and the development of a guide for assessing aquatic impacts of operating highways. The data were used to construct a simple model that expresses cumulative pollutant loadings as functions of highway segment length, average runoff coefficient, and vehicles traveling during storm periods. To assess pollutant loadings and concentrations in runoff from an individual storm, cumulative distributions were analyzed to determine the probability of specific loading and concentration values being exceeded in a given case. Bioassay studies of highway runoff indicated toxicity to aquatic life when heavy deposition of metals from high traffic volumes (> 10,000 vehicles per day) or high concentrations of metals in rainfall caused concentrations in runoff to exceed lethal levels. Draining highway runoff through grass channels 200 to 300 ft long greatly reduced concentrations of solids and metals and the consequent toxic effects. The impact assessment guide incorporates these results in a stepwise procedure for use by highway designers and environmental impact analysts in the Pacific Northwest. The guide is organized in three analysis levels, ranging from a rapid screening method intended to identify those cases with a low probability of extensive impacts (level 1) to a detailed evaluation focusing on impact mitigation (level 3). It presents methods for assessing the water-quality impacts of winter maintenance and special problems in addition to the effects of runoff from routinely operating highways.

Comprehensive studies of the characteristics, transport, and environmental effects of runoff from operating highways, as a specific component of general storm drainage, are few in number. Several investigations, however, have thoroughly treated some aspect of the subject. Early work was concerned primarily with deposition of contaminants on urban streets (_! _-})• This theme was also the subject of more recent work (~-_ § ). Sylvester and DeWalle (7) and Soderlund and Lehtinen (~) were among the first to derive pollutant mass loadings for highways, an effort supplemented by the Municipality of Metropolitan Seattle (11 10). Shaheen (11) advanced the development of pollutant loading information by vacuuming and flushing Washington, DC, area highways and statistically relating mass loadings to traffic density. Several researchers in the United Kingdom have also been active in characterizing the water quality of highway runoff (12-14). The Envirex Division of Rexn~ dCorporation and the California Department of Transportation conducted fairly comprehensive highway runoff studies at multiple sites over periods of several years. The Envirex investigation (15, 16) covered five sites in different parts of the country to represent different climatic and traffic conditions. It concluded with the development of a deposition model for predicting the accumulation of pollutants in the periods before storms and a washoff model for forecasting contaminant removal in the runoff. The California study (17) was concerned primarily with thoroughly characterizing the physical, chemical, and biological constituents in California highway runoff. A later report of this work (18) also dealt with the effects of runoff through""" bioassays in which algae were exposed to highway drainage. Thus, the literature reflects attention to the origin, characteristics, transport, effects, and modeling of highway runoff. Missing from the research record, however, is assimilation of these various results for the purposes of assessing environmental impact and applying mitigative …