San Diego County Route S21 is a south-north running road serving the coastal communities of northern San Diego County, running from San Diego in the south to Oceanside in the north. The route is signed in many places as "Historic Route 101" with the official Historic U.S. 101 shields. S21 follows the prior alignment of U.S. Route 101 through this region. The route is also called "Coast Highway" in some places as well. This route was originally designated in 1968 and is 24.74 miles long.