riveted deck truss bridge, listed on the National Register of Historic Places, carries SR 17 into Okanogan County and the Colville Indian Reservation, west of Bridgeport State Park. The highway continues north along the east bank of the Columbia River before passing through Fort Okanogan and ending at an intersection with US 97 east of Brewster near the confluence of the Columbia and Okanogan 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 annual average 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 expressway between SR 171 and Stratford Road in Moses Lake, serving an average of 19,000 vehicles, while the least busiest section of the highway was between SR 172 in Sims Corner and SR 174 in Leahy, serving an average of 570 vehicles. SR 17 between Mesa and the Moses Lake area is designated as part of the National Highway System, which includes roadways important to the national economy, defense, and mobility; and as part of WSDOT's Highways of Statewide Significance, which includes highways that connect major communities in the state of Washington. History SR 17 was established during the 1964 highway renumbering as the successor to three highways that were designated under the primary and secondary state highway system: SSH 11G from US 395 and PSH 11 in Eltopia to PSH 7 in Soap Lake, a branch of PSH 7 from Soap Lake to US 2 and PSH 2 west of Coulee City, and a branch of PSH 10 from Coulee City to US 97 and PSH 10 east of Brewster. The Coulee City–Brewster branch of PSH 10 was codified in 1931 as a branch of State Road 10 and re-codified in 1937, during the creation of the primary and secondary state highways; however, the highway was not built between Sims Corner and Bridgeport until the 1950s. The Soap Lake–Coulee City branch of PSH 7 was established in 1937 to serve as an alternate route to SSH 2F, avoiding the Grand Coulee Dam. In 1951, the PSH 10 branch between Bridgeport and Brewster was moved north of the Columbia River via the newly built Columbia River Bridge on the present route of SR