While owning and installing an aftermarket air horn or train horn is not illegal, some states have periodic inspections that your vehicle has to pass. For some of these inspections, having a train horn hooked up to your ride or, in some cases, having it wired up to be your only horn will cause you to fail inspection.

Are train horns illegal in cars?

In California, a train horn can be mounted on a car but not used. On a semi-truck, it is allowed if it is attached to the air brake system, powered by the air from the brake system, complies with FMVSS standards 121 and 106, and has the appropriate certifications.

Is it illegal to have a loud horn?

Every motor vehicle when operated upon a highway shall be equipped with a horn in good working order and capable of emitting sound audible under normal conditions from a distance of not less than 200 feet, but no horn or other warning device shall emit an unreasonable loud or harsh sound or a whistle.