The highway travels southwestward in the Lewis River valley and passes through the communities of Yale and Ariel. SR 503 enters Woodland and turns south onto Goerig Street before ending at a diamond interchange with I-5 near the confluence of the Lewis and Columbia rivers. Every year, the Washington State Department of Transportation (WSDOT) conducts a series of surveys on its highways in the state to measure traffic volume. This is expressed in terms of average annual daily traffic (AADT), which is a measure of traffic volume for any average day of the year. In 2011, WSDOT calculated that the busiest section of the highway was the SR 500 concurrency in Orchards, serving 34,000 vehicles, while the least busiest section was at the western terminus of SR 503 Spur east of Yale, serving 720 vehicles. SR 503 between Orchards and Battle Ground is designated as part of the National Highway System, which includes roadways important to the national economy, defense, and mobility. SR 503 follows the route of State Road 15, established in 1909 as the Lewis River Road, traveling east from Woodland to the Skamania county line in Columbia National Forest. The Lewis River Road was removed from the state highway system in 1919 and transferred to Cowlitz County for maintenance. Clark and Cowlitz counties contracted the Portland Bridge and Iron Company to build a bridge over the Lewis River in 1917 and the bridge was opened to traffic in September 1920 at a cost of $15,000. After the completion of the Merwin Dam downstream in 1931 caused water levels on the Lewis River to rise, the two counties completed a one-lane warren truss bridge named the Yale Bridge over the Lewis River the following year to serve the remote communities of Yale and Yacolt. During the creation of the primary and secondary state highways in 1937, State Road 15 was added back to the state highway system as part of SSH 1S, which traveled from Primary State Highway 1 (PSH 1) and U.S. Route 99 (US 99) to SSH 1U in Battle Ground and turned west to end at PSH 1 and US 99 north of Mount Vista. PSH 1U, also established in 1937, traveled south from SSH 1S in Battle Ground to SSH 8A in Orchards. The Yale Bridge was maintained by Clark and Cowlitz counties until the 1940s and had its timber approach spans replaced with steel in 1958 under maintenance of the Department of Highways.