Seattle, WA Noise Ordinances: Vehicle Noise (2026)

Section 11.84.080 sets the baseline for every motor vehicle in Seattle: a muffler in good working order and in constant operation to prevent excessive or unusual noise. The same section bars any muffler cut-out, bypass or similar device on a street or alley. Section 11.14.370 defines a muffler as a device of chambers or other mechanical designs that receives exhaust gas from an internal combustion engine and is effective in reducing the noise that results. Motorcycles carry an added numeric test under Section 11.57.280: no one can modify the exhaust to amplify the engine noise above what the original muffler emitted, and operating a motorcycle whose exhaust noise exceeds 95 decibels under SAE test procedure J1169 (May, 1998) is unlawful.

Section 11.84.070 covers engine compression brakes, the devices that slow a truck by using the engine and transmission to impede forward motion instead of the wheel brakes. Subsection A applies to motor vehicles with a declared gross weight in excess of 10,000 pounds operated on a street or alley. The driver cannot use the device unless the vehicle has an operational muffler and exhaust system that maintains noise at 83 decibels or less for vehicles manufactured after January 1, 1979, or 80 decibels or less for vehicles manufactured after January 1, 1988. The alternative is an emergency: the driver reasonably believes the device is needed to protect against an immediate threat to the physical safety of the driver or others, to protect against immediate threat of damage to property, or to reduce speed using the manufacturer's braking system when declining from an elevated roadway.

Subsection B adds a second, independent restriction. On any street where signs prohibit compression brakes, no person can use the device, with one exception: an immediate stoppage or slowing is necessary to prevent injury to persons or damage to property, or to remedy an injury that has already occurred, and friction brakes are either not available or would not have been as effective. Vehicles of a municipal fire department are exempt from subsection B whether or not they are responding to an emergency, taking part in an emergency management exercise, or rendering assistance under a mutual aid pact. Under Section 11.16.340.P, the Traffic Engineer decides which streets get the signs, and a street is eligible only if its grade is 5 percent or less for a distance of 1,000 feet.