SSH 1D was added to the state highway system in 1937. This highway started at Jct PSH 1 Anacortes Branch at Sharps Corner and headed south via the deception Pass Bridge, Oak Harbor, and Coupeville to Columbia Beach (Clinton) on the SE side of Whidbey Island. In 1964 this highway became part of SR 525.
A branch of SSH 1D was also added to the state highway system in 1937. This branch started at a junction with the SSH 1D trunk east of Keystone and headed west to Keystone. This route became part of SR 113 in 1964.
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