San Mateo County, CA Noise Ordinances: Vehicle Noise (2026)

For vehicles being operated on public streets and highways, California state law - not a county decibel ordinance - is the controlling authority, because the Vehicle Code occupies this field statewide. Vehicle Code Section 27150 requires every motor vehicle with an internal combustion engine to be equipped at all times with an adequate muffler in constant operation and properly maintained to prevent excessive or unusual noise, and prohibits cutouts, bypasses or similar devices. Vehicle Code Section 27151 prohibits modifying a vehicle's exhaust system in a manner that amplifies or increases the noise so that the vehicle exceeds the applicable noise limits. These provisions are enforced by the California Highway Patrol and the County Sheriff. The County's own Noise Control chapter (4.88) is aimed mainly at fixed noise sources measured at nearby residences and similar properties; loud vehicles parked on private property or revving in place can still implicate the General Noise Regulation (4.88.350), but moving traffic noise on public roads is addressed through the Vehicle Code.