California Code, Streets and Highways Code - SHC § 131.5

The department and any county having a park commission may enter into and carry out cooperative agreements for the grading, development, planting and maintenance of roadside areas along any State highway and within the right of way thereof and providing that the department be reimbursed for any expenditure incurred by it. Counties entering into such agreement are authorized to indicate such roadside parks and their approaches by suitable signs in the manner and to the extent provided in any such agreement. It is a misdemeanor for any person to park any house trailer in any such roadside park.