II.        Background


           The Motor Vehicle and Schoolbus Safety Amendments of 1974 directed NHTSA to issue motor vehicle safety standards applicable to school buses and school bus equipment.  In response to this legislation, NHTSA revised several of its safety standards to improve existing requirements for school buses, extended ones for other vehicle classes to those buses, and issued new safety standards exclusively for school buses. FMVSS No. 222, one of a set of new standards for school buses, improves protection to school bus passengers during crashes and sudden driving maneuvers.

            Effective since 1977, FMVSS No. 222 contains occupant protection requirements for school bus seating positions and restraining barriers.  Its requirements for school buses with GVWR's of 4,536 kg (10,000 lb) or less differ from those set for school buses with GVWR's greater than 4,536 kg (10,000 lb), because the "crash pulse" or deceleration experienced by the small school buses is more severe than that of the large buses in similar collisions.  For the small school buses, the standard includes requirements that all seating positions must be equipped with properly installed lap or lap/shoulder seat belt assemblies and anchorages for passengers².  NHTSA decided that seat belts were necessary on small school buses to provide adequate crash protection for the occupants.  For the large school buses, FMVSS No. 222 relies on requirements for "compartmentalization" to provide passenger crash protection.  Investigations of school bus crashes prior to issuance of FMVSS No. 222 found the school bus seat was a significant factor in causing injury.  NHTSA found that the seat failed the passengers in three principal respects: by being too weak, too low, and too hostile (39 FR 27584; July 30, 1974).  In response to this finding, NHTSA developed a set of requirements which comprise the “compartmentalization” approach. 

            Compartmentalization ensures that passengers are cushioned and contained by the seats in the event of a school bus crash by requiring school bus seats to be positioned in a manner that provides a compact, protected area surrounding each seat.  If a seat is not compartmentalized by a seat back in front of it, compartmentalization must be provided by a padded and protective restraining barrier.  The seats and restraining barriers must be strong enough to maintain their integrity in a crash yet flexible enough to be capable of deflecting in a manner which absorbs the energy of the occupant.  They must meet specified height requirements and be constructed, by use of substantial padding or other means, so that they provide protection when they are impacted by the head and legs of a passenger. Compartmentalization minimizes the hostility of the crash environment and limits the range of movement of an occupant.  The compartmentalization approach ensures that high levels of crash protection are provided to each passenger independent of any action on the part of the occupant.