Cal. Veh. Code § 21758

In the event any vehicle is being operated on any grade outside of a business or residence district at a speed of less than 20 miles per hour, no person operating any other motor vehicle shall attempt to overtake and pass such slow moving vehicle unless the overtaking vehicle is operated at a speed of at least 10 miles per hour in excess of the speed of the overtaken vehicle, nor unless the passing movement is completed within a total distance not greater than one-quarter of a mile.