What does compartmentalization mean? The term is at the heart of the Great Seat Belt Debate. It 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. Although not an element of compartmentalization, the UCLA researchers 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."

Other UCLA conclusions included:

Compartmentalization became the heart of the federal government's policy of school bus safety. 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.

This is why standees on school buses should never be permitted.

However, the National Transportation Safety Board (NTSB) 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 NTSB, an occupant crash protection system should be developed that would protect passengers in most accident scenarios.

In response, NHTSA updated FMVSS 207, 208, 210 and 222 to enhance compartmentalization with mandatory requirements for equipping all small Type A school buses manufactured as of September 2011 with 3-point lap/shoulder seat belts. Partly due to compartmentalization and the higher cost of larger school buses equipped with these 3-point seat belts, NHTSA only published voluntary standards for equipping school buses that weigh more than 10,000 pounds with the occupant securement systems.

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, especially those students seated behind who may not be wearing their seat belt. 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. The NHTSA final rule in 2008 also required quasi-static testing of these seats to ensure that belted as well as unbelted passengers remain protected, or compartmentalized, in the event of a crash.

Seat manufacturers CE White, IMMI/Safeguard and Takata/M2K have also solved the potential of losing passenger capacity on school buses with seat belts by creating seats flexible enough to fit two largers students, such as high schoolers, or three smaller elementary or some middle schoolers per seat.

Seat backs in school buses are also now made higher, wider and thicker than before. After the most recent final rule, seat backs will be required to be at least 24-inches high. 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.

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."

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).