The Ridge Route is a reference to the oldest routing of what was to become US Route 99 through the Tejon Pass north of Los Angeles. It ran along the "ridge" of mountains on the east side of US 99's replacment - Interstate 5. The Alternate "Ridge" Route was located West of Interstate 5. The location of the plaque is in a turnout off State Route 126 just east of I5 on the south side of the road.
The plaque reads:
The super highway of its day when opened in 1915. The Ridge Route, California’s first mountain highway, has been credited by some (for better or worse) as saving the state from being divided into two separate states. Constructed, graded, and paved at a cost of about $1,500,000, it was considered one of the most scientifically constructed mountain roads in the world. From Castaic in the south to Grapevine in the north The Ridge Route was 48 miles long and had 39,441 degrees of curves. Roughly equating to 110 complete circles. The strictly enforced speed limit was 15 MPH. The road was replaced by the alternate Ridge Route (later known as US 99) in 1933, which in turn was replaced by I-5 in the 1960’s.