This paper investigated the highway stormwater quality at two Texas cities-Austin and College Station. Two highways with high average daily traffic were monitored using passive stormwater samplers for collecting first-flush runoff during a 16-month period. Detailed traffic and weather data were collected at College Station sites, but only weather data were obtained at Austin sites. A stepwise regression analysis on College Station data identifies the antecedent dry period (ADP) as the most significant predictor of pollutant concentration. Specifically, the College Station data show an unexpected result that pollutant event mean concentrations significantly decrease with increasing ADP for all analyzed pollutants. However, the runoff concentrations observed in Austin were not significantly correlated with ADP. The result from College Station data provides a different insight to the pollutant buildup and removal process on highways. Conceptual highway pollutant buildup and removal models are proposed for generating further discussion and research interest.