What does compartmentalization mean? The term is at the heart of the Great Seat Belt Debate. The term was coined during the 1967 and 1972 Series 1 and Series 2 "School Bus Passenger Protection" collision tests conducted at the University of California at Los Angeles. It is at the heart of the federal government's policy of school bus safety and is outlined in great engineering detail in The concept of compartmentalization envisions children riding in a cocoon or compartment surrounded by an energy-absorbing, passive occupant protection system. Some industry experts use the metaphor "egg carton" as in cushioning the eggs inside, to describe compartmentalization.

    Under the compartmentalization concept, seat backs in school buses are made higher, wider and thicker than before. All metal surfaces are covered with energy-absorbing padding. This structure must pass rigid test requirements for absorbing energy, such as would be required if a child's body were thrown against the padded back. The equivalent of a seat back, called a barrier, is placed in front of the first seat at the front of the bus.

    In addition to padding, today's seats also must have a steel inner structure that bends forward to help absorb energy when a child is thrown against it. The steel frame must give just enough to absorb the weight of the child in the seat behind. Also, of course, the seat is required to be anchored to the floor strongly enough that it will not pull loose during this bending action, or during collisions. Federal regulation requires each passenger seat to be anchored to the school bus floor and withstand 15,000 lbs. pulling force per seat. The floor itself must be strong enough that it will not be bent or torn by the pulling action of the seat anchors.

    Finally, seat backs cannot be farther apart than a distance that is deemed safe. Clearly, if the seats are too far apart, the child could be thrown too far before being cushioned and/or could be thrown outside the compartment altogether. Following further research by the federal government, 24 inches spacing was established as the optimal distance between school bus seats.

    "In compartmentalization the crash forces are absorbed by the vehicle structure which is designed to protect the occupant," said Charles Hott. NHTSA's school bus specifications engineer. "In an occupant restraint system in general passenger vehicles the crash forces are absorbed by the body of the occupant."

    Current design of school buses allows the seat back to bend, thereby reducing the crash forces absorbed by the occupant. However, this principle brings into conflict the flexible seatback requirement of FMVSS 222 which can easily accommodate 2-point lap belts, with the occupant protection requirement of FMVSS 208. FMVSS 208 requires occupant restraint anchorages to be attached to the vehicle structure thereby allowing it to to accomodate 3-point safety belts in general passenger vehicles. So while FMVSS 222 requires bending at 2,400 lbs. force to enable compartmentalization to work. FMVSS 208 requires seat backs to withstand 3,000 lbs. force to provide the strength for the shoulder belt.

    It is important to note that the National Transportation Safety Board (NTSB), has found that compartmentalization is "incomplete," and "does not protect passengers during lateral impacts with vehicles of large mass, in rollovers and from ejection."  According to the Safety Board, an occupant crash protection system should be developed that would protect passengers in most accident scenarios.

    Presented here is a summary of the conclusions drawn from both series of tests by UCLA researchers, plus further understandings of the concept that evolved during NHTSA rulemaking, and general usage over the past two decades. The editors of School Transportation News are solely responsible for this summary:

• High back seats of 28 inches or more greatly contribute to the compartmentalization of passengers thereby reducing the chances of injuries sustained by passengers being hurled against one another, regardless of their size. Seatback height for school buses should not be less than 28 inches."
  
• Lap belts should not be used with low seat back seats as this would lead to head and chest injuries caused by belted passengers rotating forward and striking the upright backrest ahead.
  
• School bus seats should not be provided with rigid protruding structures such as handgrips, handrails or similar injury producing fixtures.
  
• Lap belts would provide substantial additional protection if used in combination with high-back seats equipped with additional efficient padding on the rear panel of the backrest ahead.
  
• Standees should not be permitted.
  
• Seat belts are not recommended for school buses equipped with seats with hard surfaces, a metal bar along the back side of the top of the frame ahead, weak seat frames, or low-back 24 inch seats.

    Although not an element of compartmentalization, the UCLA researchers who conducted the 1967 tests on school buses concluded that after high back seats, next in importance to school bus passenger collision safety is the "use of a three-point belt, a lap belt or other form of effective restraint."

Source: School Bus Passenger Protection; Institute of Transportation and Traffic Engineering, University of California at Los Angeles, by Severy, Derwyn M.; Brink, Harrison M. and Baird, Jack D. (Los Angeles, CA 1967).