County Route S18 (CR S18) is a county highway in the US state of California in Orange County. The route follows El Toro Road and Santiago Canyon Road which proceeds in a boomerang-like pattern from State Route 133 in Laguna Beach to State Route 55 near Orange. CR S18 traverses as a loop around the urban areas of Orange County and cuts through the Santa Ana Mountains. The road is one of four county routes in Orange County that are signed in areas nearby the route, such as southbound Interstate 5 and southbound State Route 133. It is also noted to be the longest county route in Orange County and is the only major route that allows motorists to drive through, in, and out of the Santa Ana Mountains.