State Route 525 (SR 525) is a state highway located in Snohomish and Island counties in the western region of the U.S. state of Washington. SR 525 begins at an interchange with Interstate 5 (I-5) and I-405 in Lynnwood and travels north to SR 99 as a four-lane controlled-access freeway. From Lynnwood, the highway serves Mukilteo and becomes the terminus of SR 526 before taking its ferry route to Clinton on Whidbey Island. SR 525 traverses the island's interior as part of the Whidbey Island Scenic Byway before the designation ends at an intersection with SR 20 south of Coupeville. SR 525 was established during the 1964 state highway renumbering as the successor to Secondary State Highway 1D (SSH 1D) on Whidbey Island and SSH 1I in Mukilteo and Lynnwood, themselves established in 1937. The highway, at its codification in 1970, traversed the north–south length of Whidbey Island and ended at SR 536 near Anacortes until it was replaced by SR 20 in 1973. SR 525 was converted to its present freeway in Lynnwood during the 1980s and widened to four lanes during the late 1990s, including an overpass over SR 99. The highway also has a spur route in Mukilteo that has connected SR 525 to SR 526 via Paine Field Boulevard since it was signed in 2001. Route description SR 525 begins at the Swamp Creek Interchange with I-5, also serving as the northern terminus of I-405, located in Lynnwood in southern Snohomish County. The four-lane controlled-access freeway travels north past Alderwood Mall and a partial cloverleaf interchange with Alderwood Mall Parkway, which serves the eponymous mall. SR 525 continues north under overpasses carrying 164th Street and 148th Street before reaching its partial cloverleaf interchange with SR 99, where the freeway ends. The highway becomes the four-lane Mukilteo Speedway and travels northwest into the city of Mukilteo, serving its commercial and industrial areas located south of Paine Field. SR 525 intersects its spur route, which travels north as Paine Field Boulevard towards the Boeing Everett Factory, before it serves as the western terminus of SR 526. The highway gains a ferry holding lane on its northbound shoulder as it approaches the Mukilteo ferry terminal, where the designation of SR 525 is carried onto the Whidbey Island Ferry across Possession Sound. The terminal is east of Mukilteo Lighthouse Park and adjacent to a train station served by Sounder commuter