The 11.00-mile-long (17.70 km) 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.

[1] The highway travels east through Downtown Ridgefield, passing View Ridge Middle School, before crossing over Gee Creek.

[10][11] Every year, WSDOT conducts a series of surveys on its highways in the state to measure traffic volume.

[7] As of July 6, 2024, the Northern Section between the western Ridgefield City Limits and the eastern terminus at Interstate 5 was transferred to the City of Ridgefield through the Route Jurisdiction Transfer process, enacted by House Bill 2318, unanimously approved by the 68th Legislature 2024 Regular Session, and signed into law by Governor Jay Inslee on March 13, 2024.

SSH 1T was created in 1937 during the formation of the primary and secondary state highways,[12] traveling on a 18.86-mile-long (30.35 km) route north from Primary State Highway 1 (PSH 1) and U.S. Route 99 (US 99) in Vancouver through Felida and Ridgefield to PSH 1 and US 99 at Pioneer.

[24] On July 14, 2015, WSDOT permanently closed the northern 1+1⁄3-mile-long (2.1 km) of SR 501's southern section due to the Columbia River reclaiming the highway.