Seattle, WA Noise Ordinances: Vehicle Noise (2026)

Seattle Municipal Code Chapter 11.84 sets the city's vehicle noise rules. SMC 11.84.080 requires "a muffler in good working order and in constant operation to prevent excessive or unusual noise" on every motor vehicle operated on a street or alley, and flatly bans muffler cut-outs, bypasses, or similar devices, there is no decibel test for ordinary passenger vehicles, just the working-muffler/no-cutout standard, enforced under RCW 46.37.390(1) as adopted locally.

Heavier vehicles get a separate, numeric rule. SMC 11.84.070 applies to motor vehicles over 10,000 pounds declared gross weight equipped with engine compression brake devices (the "Jake brake" systems that use engine and transmission compression to slow a truck). A driver may not engage the device unless the truck's muffler and exhaust system holds noise to 83 decibels or less for vehicles manufactured after January 1, 1979, or 80 decibels or less for vehicles manufactured after January 1, 1988, or unless the driver reasonably believes an emergency requires it to avoid an immediate safety or property threat, or to control speed descending an elevated roadway. Separately, subsection B lets the city post signs on specific streets prohibiting compression-brake use outright, subject to the same emergency exception; municipal fire department vehicles are exempt from that sign-based restriction. Both the muffler standard and the compression-brake decibel rule trace to RCW 46.37.390(1) and RCW 46.37.395, incorporated into the municipal code by Ordinance 122742 (2008) and its predecessor Ordinance 108200 (1979).