A siren . . . or a horn . . . or 4-way flashers. A red light . An air horn . . . or an amber light . An electric switch that works by air pressure A computer. A motion sensor in the wheels.
Electric relay switches.
Front wheel brakes. So you can move the vehicle in an emergency. To simplify servicing the air brake system. So that the brakes will release quicker. Two separate air brake systems with a single set of brake controls.