Abstract: An inertial switch device, for installation in an electrical circuit of an automotive vehicle, to signal excessive rates of acceleration, deceleration and turning for the purpose of preventing abusive driving which wastes gasoline. The switch device comprises a housing having a closed chamber with a concave bottom surface containing a globule or drop of mercury and a plurality of circular contacts supported therein so that the drop of mercury successively contacts them when a momentum force causes the drop of mercury to climb a side of the dish. The circular contacts are of increasingly larger diameter as the drop climbs the side of the dish with increasing inertia and are arranged in circuit-closing relation to a warning signal device, either audible or visible.