Usually steering is powered by a hydraulic pump driven by the engine's serpentine belt. Automatic transmissions have torque converters, which are one-way (the engine can turn the wheels, but the wheels can't turn the engine). As soon as the engine dies, the serpentine belt stops, hydraulic pressure drops, and steering gets hard.<p>Some newer models (usually hybrids) power the hydraulic pump with an electric motor, and a few cars have purely electric power steering (just an electric motor, no hydraulics). In these types, a kill switch could be set up to stop only the engine, not the electrics. The battery could power the car long enough to stop safely.