The top priorities for design of power-assisted steering, especially for heavy commercial vehicles, are traffic safety and driving comfort. Legal limits of steering-assistance attempt to define safety standards; more power assistance eases control of the vehicle, but excessive assistance leads to lack of "feel" for the road.Analysis of steering forces and tests of vehicles, both conducted for varying driving speeds and conditions, indicated two factors which are critical for the evaluation of sudden failure of power-assistance: the steering force, and the assistance factor. In general, the assistance component is most needed at low speeds. Results identify driving condition limits for maintenance of vehicle control upon loss of steering assistance. Nevertheless, power-assistance must be limited in order to reduce the surprise factor should a system failure occur. Current power-assistance designs which reduce the assistance factor as driving speed increases are well suited to meet these needs.