While California law does not specifically require buses and semi-trailer trucks to be equipped with mud flaps, it does require some kind of device to perform the same function as flaps. Flaps are required under state law on vehicles that transport trash, ashes and other such material. Mud flaps attached to vehicles hauling cargo, people or garbage must conform to state requirements regarding their condition and mounting.
Mud flaps, which are also known as mud guards, splash flaps or just simply flaps, are generally large, rectangular sheets of rubber suspended behind the tires of buses, full-size pickup trucks, semi-trailer trucks and other large utility vehicles. Some flaps are emblazoned with company logos, the Yosemite Sam cartoon character with the words "Back off!" or the silhouette of a shapely woman whose hair blows back in the wind.
Working in combination with fenders, mud flaps protect the vehicle, its passengers, other vehicles and pedestrians from mud, water and other flying debris that can be kicked up into the air by rotating tires.
"No person shall operate any motor vehicle having three or more wheels, any trailer, or semitrailer," states California Vehicle Code Section 27600, "unless equipped with fenders, covers, or devices, including flaps or splash aprons, or unless the body of the vehicle or attachments thereto afford adequate protection to effectively minimize the spray or splash of water or mud to the rear of the vehicle and all such equipment or such body or attachments thereto shall be at least as wide as the tire tread." The law became effective Nov. 23, 1970.
"This section does not apply to those vehicles exempt from registration, trailers and semitrailers having an unladen weight of under 1,500 pounds," California Vehicle Code Section 27600 further states, "or any vehicles manufactured and first registered prior to January 1, 1971, having an unladen weight of under 1,500 pounds."
California Vehicle Code Sections 23114 and 23115, which cover "Spilling Loads and Damage to Highway," require "splash flaps behind every tire or set of tires." Those sections apply to "any vehicle transporting garbage, trash, rubbish, ashes, etc." According to the sections, "Any person who willfully or negligently damages any street or highway is liable for the cost of repairing the road or any sign, signal, guard rail, or other facility that is damaged. The liability may include the cost of removing debris from the roadway."
In Chapter 5 of the California Commercial Vehicle Handbook, which covers employer testing of driver performance, the "Pre-Trip Inspection Scoring Criteria" includes the following with regards to splash guards on commercial trucks: "If equipped, check that splash guards or mud flaps are not damaged and are mounted securely." The exact same language is used in the later section on bus inspections.