Speed and severity trigger for an active device of a vehicle

A method of determining the category of action of an active device of a vehicle comprises repeatedly measuring a vehicle parameter indicative of a change of vehicle state, determining a rate of change of the vehicle parameter and determining a modified vehicle parameter by reference to said rate of change. A threshold is applied to said modified vehicle parameter and a trigger signal is generated for an active device of the vehicle in response to the threshold being reached and in accordance with a severity of said vehicle parameter, which may be vehicle speed related.