Instead of terminating in Los Angeles, Highway 101 used to travel all the way south through San Diego to the United States–Mexico border in San Ysidro. However, this part was deleted in the late 1960s in favor of Interstate 5. The only remnant of the old route is a mileage sign at the Santa Barbara/Ventura County line, which lists the distance to San Diego, even though Highway 101 ends in Los Angeles.
The old alignment in San Diego County from Oceanside to Del Mar is now known as San Diego County Route S21.
It is signed unofficially in many places as "Historic Route 101".
Before I-5's implementation in the 1960s, long sections of 101 in this
area (through USMC's Camp Pendleton and south to the northern reaches of
San Diego) were three lanes, with the center lane being a 'passing
lane' for both northbound and southbound drivers. Horrific and fatal
head on collisions were not uncommon. In newspaper accounts, it was
often referred to as the "suicide lane".
Significant portions of 101 were originally known as the Royal Road or El Camino Real.
The name, El Camino Real, continues in widespread use from South San
Francisco to San Diego for surface routes, most of which are close to,
and parallel to 101.
The primary control city
that is listed on freeway signs along northbound 101 through the
Central Coast region is San Francisco. Although San Jose surpassed San
Francisco population decades after the highway was built, there has been
no push to change all the signs.