 Prior to the 1964 highway renumbering, SR 501 was designated as Secondary State Highway 1T (SSH 1T), established in 1937 and re-aligned to serve the Port of Vancouver in 1963.The southern section of SR 501 and the Erwin O. Rieger Memorial Highway begins as the four-lane Mill Plain Boulevard at a diamond interchange with I-5 in Downtown Vancouver.The highway travels west through Downtown Vancouver and splits into a one-way pair on Mill Plain Boulevard and 15th Street before rejoining as Mill Plain Boulevard.