 The road once extended east from Portage Creek to the banks of the South Fork Stillaguamish River, but this section was removed from maps by 1940.As retailers moved into the Smokey Point area, sections of the road were widened and improved in the 1980s with contributions from private developers.SR 531 was designated as a state highway during the 1991 legislative session, but it was not transferred to state control until April 1992.