The highway then heads northward through Siskiyou County, roughly lying parallel to the Shasta River.

The road is also roughly aligned with nearby Interstate 5 as it passes through hilly terrain.

The road ends at State Route 96, which continues northward to meet up with Interstate 5.

[4] Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment of U.S. Route 99 as it existed at that time, and do not necessarily reflect current mileage.

[1] Segments that remain unconstructed or have been relinquished to local control may be omitted.