California Code, Streets and Highways Code - SHC § 219

(a) Safety roadside rests should be provided so that, in combination with other stopping facilities, there shall be facilities available at intervals of approximately one-half hours normal driving time.

(b) On highway entrances to large metropolitan areas, safety roadside rests may be provided.

(c) On high-volume highways consisting of four or more lanes, safety roadside rests should be provided on both sides of the highways; on all other highways only single safety roadside rests should be provided.

(d) Notwithstanding the provisions of subdivision (a), on high-volume highways more safety roadside rests may be planned at strategic locations where they appear to be necessary or desirable.