As a charter member and chairman, Hewitt had the longest tenure of any member of the State highway Commission (1964-1975). He drew on his experiences as president of the Detroit Bank and Trust Co., and a long career in banking and finance, to improve the efficiency and strengthen the financial controls of the Department of State highways and Transportation. He also played an important role in the transition of the Department from a highway agency to one with responsibilities in all transportation modes, from aeronautics to rail and public transportation. Under his leadership, the commission took many significant actions to maintain Michigan's high ranking in transportation, ranging from expanded planning efforts to improved pre-qualification processes for highway contractors.