The northernmost section of the road, along Lewis Street in downtown Monroe, was paved by the city government in 1912 and connected to a bridge over the Skykomish River that was rebuilt as a steel structure in 1915. The King County government followed suit with several state-funded paving projects on the eastern highway in the 1910s and 1920s. By the mid-1920s, the paved Snoqualmie Valley highway extended south from Monroe to the King–Snohomish county line north of Duvall, while the remainder was described as an improved road with varying quality. The road was incorporated into the state highway system in 1937 as Secondary State Highway 15B (SSH 15B), connecting Primary State Highway 15 (PSH 15) in Monroe to PSH 2 and U.S. Route 10 (US 10) in Fall City. The state legislature approved funds to pave the entire highway in 1937 and oiling of the road from Carnation to Duvall was completed two years later. The final paving projects were approved in 1945 and completed over the following three years alongside the replacement of sixteen wooden bridges. The state also straightened sections of SSH 15B near Fall City in the 1940s and through the Tualco Valley south of Monroe in the 1950s. The Lewis Street Bridge over the Skykomish River in Monroe was replaced by the state in 1957 at a cost of $526,000.