SR 118 crosses the Santa Susana Pass and the northern rim of the San Fernando Valley along its route.

[2] The eastern section is an urban freeway that starts in the cities of Moorpark and Simi Valley, and ends in Los Angeles.

The SR 118 freeway initially travels north, but quickly turns east, passing near Moorpark College, before entering the Simi Valley city limits.

[9] It was renamed in honor of Ronald Reagan, the 33rd Governor of California and the 40th President of the United States, due to the location of his presidential library in Simi Valley.

[10] SR 118 used to extend past I-210 on Foothill Boulevard through Sunland-Tujunga, La Crescenta and La Cañada, then across the Arroyo Seco into Pasadena, where SR 118 ran on Lincoln Avenue and Fair Oaks Avenue, ending at Colorado Boulevard (US 66 Alternate).

As a result of the Northridge earthquake in January 1994, a section of the highway between I-405 and I-210 was closed for over one month while damage to an overpass was repaired.

The rationale for choosing this name for State Route 118 is that the western end of the highway, at the time the bill was passed, is very close to the Ronald Reagan Presidential Library.

This appears to have been roughly planned to run primarily along Big Tujunga Canyon between Foothill Boulevard and Los Angeles County Route N3, Angeles Forest Highway, which is the current traversable routing for unconstructed SR 249.

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