California Code, Streets and Highways Code - SHC § 954

All county highways which for a period of five consecutive years are impassable for vehicular travel, and on which during such period of time no public money is expended for maintenance, are unnecessary highways. The board of supervisors of any county on its own motion, or on the petition of any interested taxpayer of the county, may designate such county highway a stock trail. The board of supervisors shall cause notices to be posted upon such stock trails, and also at the entrance of such stock trails, directing all persons to drive all untethered stock thereon.

After a stock trail has been established or designated as provided in this chapter, the county is not liable for death or injury to a vehicle owner or operator or passenger, or for damage to a vehicle or its contents, resulting from a dangerous condition of the stock trail.

Such stock trails shall not be included in the term “maintained mileage of county roads” as that term is used in Chapter 3 (commencing with Section 2100) of Division 3.