Cal. Vehicle Code § 21650

Upon all highways, a vehicle shall be driven upon the right half of the roadway, except as follows: (a) When overtaking and passing another vehicle proceeding in the same direction under the rules governing that movement. (b) When placing a vehicle in a lawful position for, and when the vehicle is lawfully making, a left turn. (c) When the right half of a roadway is closed to traffic under construction or repair. (d) Upon a roadway restricted to one-way traffic. (e) When the roadway is not of sufficient width. (f) When the vehicle is necessarily traveling so slowly as to impede the normal movement of traffic, that portion of the highway adjacent to the right edge of the roadway may be utilized temporarily when in a condition permitting safe operation. (g) This section does not prohibit the operation of bicycles on any shoulder of a highway, on any sidewalk, on any bicycle path within a highway, or along any crosswalk or bicycle path crossing, where the operation is not otherwise prohibited by this code or local ordinance. (h) This section does not prohibit the operation of a transit bus on the shoulder of a state highway in conjunction with the implementation of a program authorized pursuant to Section 148.1 of the Streets and Highways Code on state highways within the areas served by the transit services of the Monterey-Salinas Transit District or the Santa Cruz Metropolitan Transit District.