SSH 1B, added to the state highway system in 1937, began at Jct PSH 1 (US 99, now I-5) in Bellingham and ran north via Lynden to the Canadian Border. An east-west branch, starting at a junction with the SSH 1B trunk just north of Lynden and heading east to Jct SSH 1A a few miles SW of Sumas, was added in 1957. This east-west branch had been part of SSH 1A from 1937 to 1953.
The north south route of SSH 1B was posted as Alt US 99 from 1954 to 1969. This route is now SR 539, the east-west branch, SR 546.
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