One treatment practice for storm water is detention of the initial part of the runoff that is considered to contain the highest concentration of pollutants. This study has evaluated the concentration of total suspended solids (TSS) in 44 consecutive runoff events from a highway watershed. The effluent TSS standard for wastewater of 60 mg/l applied in the EU was used to assess the required treatment. In 35 of the runoff events the TSS partial event mean concentration exceeded 60 mg/l for the duration of the runoff event. Thus, a partial capture of the runoff volume should not be used as a treatment option in similar conditions that prevailed in this study.