An extensive section of this highway (over 600 miles), from approximately Stockton, California to Vancouver, Washington, follows very closely the track of the Siskiyou Trail.
By 1972, US 99 was completely decommissioned with the completion of I-5 in Washington and California, but the highway's phasing out actually began July 1, 1964 due to the passage of Collier Senate Bill No. 64 on September 20, 1963.