Storm water discharged from highways throughout the United States and other countries represents a significant component of nonpoint source pollution (NPS). Its impact on water quality has traditionally been aggregated with other NPS originating from watershed runoff [1]. State transportation agencies in the United States are required to secure a storm water permit that is mandated by the National Pollution Discharge Elimination System (NPDES). NPDES storm permits that have been issued to each of the state transportation departments generally require the implementation of a storm water program to characterize pollutant loadings of highway runoff, and a retrofit program to mitigate its impact on receiving streams. In an effort to develop a comprehensive watershed management program with shared responsibilities among state agencies, highway runoff must be quantified and treated as an identifiable contributor to the overall NPS pollutant loads, rather than aggregating it with watershed runoff. Technical guidance is needed to properly char- acterize and prioritize pollutant loadings associated with highway runoff.