Water quality of highway stormwater runoff from an ultra-urban area was characterized by qualitatively and quantitatively defining flushing and determining the event mean concentration (EMC) and annual pollutant loadings for TSS, total phosphorus (TP), NO3-, NO2-, TKN, Cd, Cu, Pb and Zn. Characterization aids in evaluation of the performance of a retrofit bioretention facility. Furthermore, this research contributes to the limited data available on recent characterization of runoff from urban areas. Mean EMC's in mg/L were 0.04, 0.11, 0.22, 1.18, 420, 3.4, 0.96, and 0.56 for Cd, Cu, Pb, Zn, TSS, TKN, NO3+2-, and TP. Median values for the mass flushed in the first 25% of runoff were greater than the mass flushed in a 25% portion beyond the first, except for TSS and Cd. The mass in later 25% portions were greater than in the first 25% in 17% of the events for all pollutants.