When first posted as such in January 1964, SR 515 started at Jct SR 516 on the east side of Kent and headed north via Benson Hill to Jct SR 405 in Renton. In 1965, the Renton Freeway, I-405, opened to traffic from the Southcenter area to Sunset Blvd in Renton. Southcenter itself did not open until 1968. SR 515 was extended a few blocks north via former SR 405 to Jct SR 900 in Downtown Renton. In the 1970s, the route of SR 515 was moved from the 2 lane Benson Rd to a new 4 lane route called Benson Drive. During the 1980s and early 90s, WSDOT widened the remainder of SR 515 from the south end of Renton to Jct SR 516 in Kent to 4 lane, mostly with a center 2-way left turn lane. Today, SR 515 is 7.86 miles long.
Before 1964, SR 515 had been SSH 5C.
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