The basic purpose in spacing school bus seats so closely is to contain the child in a cushioned compartment with only a minimum amount of space between energy-absorbing surfaces.
After extensive research during the 1970's, the Department of Transportation (DOT) and its agency, the National Highway Traffic Safety Administration (NHTSA) determined that the safest and most practical arrangement for school bus seating would be a "compartmentalization" concept. Accordingly, the new safety regulations established in 1977 included this requirement among many other improvements made that year. Under the compartmentalization concept, seat backs in school buses are made higher, wider and thicker than before. All metal surfaces are covered with foam padding. This structure must then pass rigid test requirements for absorbing energy, such as would be required if a child's body were thrown against the padded back. In addition, 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 springs and bends forward to help absorb energy when a child is thrown against it. The steel frame must "give" just enough to absorb the child in the seat ahead. Also, of course, the seat is required to be anchored to the floor so strongly it will not pull loose during this bending action. The floor itself must be so strong that it will not be bent or torn by the pulling action of the seat anchors.
Finally, the requirement is added that seat backs can be no farther apart than a distance that is deemed safe. Clearly, if the backs were too far apart, the child could be thrown too far before being cushioned and/or could be thrown outside the compartment altogether. Today's rules call for a seat back to be no farther than 24" away from a defined point in the middle of a child's abdomen (the seat reference point).