The objective of this study was to evaluate correlations between annual average daily traffic (AADT) and storm water runoff pollutant concentrations generated from California Department of Transportation (Caltrans) highway sites. Analyses of data collected from the Caltrans Cyear (1997-01) highway runoff characterization program revealed that, in general, pollutant concentrations from urban highways were higher than those found from non-urban highways. For a limited number of pollutants, however, the concentrations from norrurban highways were found to be higher than the concentrations from urban highways. No direct linear correlation was found between highway runoff pollutant event mean concentrations (EMCs) and AADT. However, through multiple regression analyses, it was shown that AADT has an influence on most highway runoff constituent concentrations, in conjunction with factors associated with watershed characteristics and pollutant build-up and wash oft The other noticeable factors shown ' (530) 7537030, e-mail: claua@lrr~a.corn to influence the accumulation of pollutants on highways were antecedent dry period, drainage area, maximum rain intensity, and land use.Keywords: Annual average daily traffic (AADT), highway runoff, linear regression model, multiple regression model, and pollutants.The California Department of Transportation (Caltrans) is engagd in a multi-year program of research and monitoring pertaining to the environmental effects of stormwater quality from transportation facilities. Part of Caltrans storm water quality research and monitoring program involves the characterization of highway runoff (Kayhanian et al., 2001). These monitoring studies were principally undertaken (i) to comply with the statewide National Pollution Discharge Elimination System (NPDES) storm water permit requirements, (ii) to address legal requirements, (iii) to aid in developing new treatment systems, (iv) to develop runoff load models, and (v) to fill data gaps in stormwater runoff characterization for statistical analysis. The information presented in this paper is based on a 4-year highway stormwater runoff characterization study that was undertaken during the 1997-01 rainy seasons from October through April.Caltrans monitoring data are analyzed on a regular basis to assess runoff characteristics. One question that is frequently asked i s whether a correlation exists between annual average daily traffic (AADT) and the concentrations of highway runoff pollutants. The current paper addresses this issue.
METHODS
Sampling ProceduresRepresentative highway sites and storm events were selected for event-based monitoring. There are a wide range of parameters that can potentially affect the quality of stormwater discharges including geographic location, climatic/ecologic conditions, hydrologic conditions, land use, and AADT. The highway sites were selected to represent the full range of physical parameters. In addition, the sites were selected as potential monitoring sites based on the ability of the sampling t...
