This provision is both a prohibition and an "emission" control. it limits to use for safety warning only.. It limits the sound level to a specific level at a prescribed distance. This provision is intended to limit horn use to safety and to limit the use of excessively loud air horns or "Rumbler" or "Howler" horns. California prohibits a person operating a motor vehicle to wear a headset or earplugs on both ears. Oregon (Section 820.370) prohibits signaling sound when an emergency vehicle is stationary or returning from an emergency