SSH 1T was added to the state highway system in 1937. This route started at Jct PSH 1 (US 99) in Vancouver and headed westerly and northerly via Sara to Ridgefield, then easterly, ending at Jct PSH 1 (US 99) at Pioneer. When US 99 was shifted from its route via Pioneer to the present route of I-5 west of Pioneer in the early 1940s, SSH 1T from new US 99 to old US 99 was dropped from the state highway system. When US 99 was shifted from its route through Downtown Vancouver to the present route of I-5 just east of downtown in the mid 1950s, SSH 1T was extended from old US 99 to new US 99.
1963 legislation realigned SSH 1T between the west side of Vancouver and Ridgefield. The route via Sara was dropped from the state highway system. A new route via Lower River Road and a new road to Ridgefield was added. Most of this new road has not yet been built as of 2001. SSH 1T is now designated as SR 501.
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