The Guide Meridian, named for the guide meridian that it follows while traveling due north–south, was originally a plank road constructed in the late 1880s. It was replaced with a gravel road in the 1910s and a paved highway later that decade by the Whatcom County government. The Guide Meridian was absorbed into the state highway system and designated as Secondary State Highway 1B (SSH 1B) in 1937, which was later supplemented with the creation of U.S. Route 99 Alternate in 1952. Both designations were replaced with SR 539 in 1969 following the completion of I-5 in Bellingham.