Abstract: The severity of a crash is discriminated from a measure of relative motion of first and second elements of a vehicle, wherein an initial time is recorded when a variable point in the associated phase space of relative velocity and relative displacement crosses a first threshold boundary, and a safety restraint actuator is actuated if a subsequent time when the variable point crosses a second threshold boundary is within threshold period following the initial time. In accordance with another aspect, a type of impacting object is discriminated from a first measure of relative motion at a first location, and a crash severity is discriminated from a second measure of relative motion at a second location, wherein the relative motion at the first location is unipolar for one type of crash and bipolar for another, and the relative motion at the second location is unipolar for both types of crashes.