The earliest observation of pollutant accumulation and wash-off from street surfaces can be traced back to the work of Sartor and Boyd [26], followed by the National Urban Runoff Pollution study [1] and others [20]. These earlier studies were intended to characterize storm water quality from different land uses within an urban watershed. Pollutant loadings due to highway runoff were included as part of the overall urban runoff loadings. The effects of traffic conditions during and prior to storm runoff are usually excluded from these runoff models because their purpose was to predict urban storm water loads in general. As stated earlier, state transportation agencies are in need of planning tools to characterize pollutant loads from highway runoff sites, rather than aggregating its effects with the overall watershed runoff. Numerous types of models have been investigated to predict highway runoff quality [27]. Two significant studies employing more comprehensive datasets have been carried out to help support these needs, as summarized below.