To operate existing roadways and construct new ones, departments of transportation and other transportation agencies must meet increasingly stringent regulations for stormwater runoff. The objective of this study was to evaluate the benefit of amending roadway soils with biochar, which is postulated to reduce nutrient loading and stormwater runoff volume because of its high internal porosity and surface area, high cation exchange capacity, and ability to alter soil hydraulic properties. After screening biochars, experiments were conducted at lab-scale, pilot-scale, and field-scale to evaluate the impact of a wood-derived biochar on hydraulic properties of three representative biochar-amended soils and resulting reduction of stormwater volume. Field-scale experiments were conducted with support from the National Fish and Wildlife Foundation: biochar was amended at 4% mass fraction to a sandy loam in a roadway filter strip along a four-lane divided highway in Delaware. Biochar amendment improved hydraulic properties of all amended soils: available water content was increased by 20-70% depending on soil. At the field site, biochar amendment reduced stormwater runoff volume and peak flow rate by 84 and 77%, respectively, over 74 storm events in 2016/2017. At this field site, 0.12 acres of biochar-amendment was needed to treat 1-acre impervious with approximately 83% removal of nutrients and sediments at a cost of ~ $31,700 per impervious acre treated. Future work should examine the longevity of biochar amendment; the utility of using less biochar, e.g., 2% application rate, to achieve the same benefit; and development of models to predict the effects of biochar amendment on stormwater treatment for a wide range of applications.