Cal. Veh. Code § 23127
No person shall operate an unauthorized motor vehicle on any state, county, city, private, or district hiking or horseback riding trail or bicycle path that is clearly marked by an authorized agent or owner with signs at all entrances and exits and at intervals of not more than one mile indicating no unauthorized motor vehicles are permitted on the hiking or horseback riding trail or bicycle path, except bicycle paths which are contiguous or adjacent to a roadway dedicated solely to motor vehicle use.
For the purpose of this section "unauthorized motor vehicle" means any motor vehicle that is driven upon a hiking or horseback riding trail or bicycle path without the written permission of an agent or the owner of the trail or path.
This section does not apply to the operation of an authorized emergency or maintenance vehicle on a hiking or horseback riding trail or bicycle path whenever necessary in furtherance of the purpose for which the vehicle has been classed as an authorized emergency vehicle. Any person who violates this section is guilty of a misdemeanor.
Ca. Veh. Code § 23127