School buses have a unique history of design and construction that over many years has proven that the larger school buses are very crashworthy for their student passengers. The National Highway Transportation Safety Administration (NHTSA) has enacted many Federal Motor Vehicle Safety Standards (FMVSS) of which at least 55 apply to school bus construction and crashworthiness. The FMVSSs as contained in Title 49 CFR Part 571, apply specifically to school bus occupant protection including FMVSS 220 for School Bus Rollover Protection, which establishes performance requirements for reinforcing the school bus body structure to withstand forces encountered in rollover crashes. FMVSS 221 addresses School Bus Body Joint Strength and establishes performance requirements for the strength of the body panel joints in the school bus and how the bus should perform in a collision. FMVSS 222--School Bus Passenger Seating and Crash Protection establishes performance requirements for school bus passenger seating and restraining barriers.

Both Type A and B vans or buses are under 10,000 GWVR and must have seatbelts for occupant protection. Neither type relies on compartmentalization as identified in FMVSS 222--School Bus Passenger Seating and Crash Protection. Type C or conventional school buses having a seating capacity of up to 72 elementary students or 41 middle or high school pupils must comply with FMVSS 222. This bus type consists of a body installed upon a flat-back cowl truck chassis with a GWVR of more than 10,000 pounds and an engine located forward of the windshield. Empty Type C buses weigh in at 22,000-25,000 pounds. The Type C entrance door and driver's seat are aft of the front axle. Some Type C buses have a shorter chassis and body with fewer seat rows and a reduced seating capacity. Most have the standard 13 rows of seats on both sides of the aisle with a narrow 16" aisle separating 39" wide seats. Seat rows are set closely as possible from the back of the bus forward to assure so that compartmentalization will better protect students in a collision. The only seat belt and shoulder harness on Type C is for the driver. Compartmentalization is assumed to protect all of the students on board.

Still larger Transit-style or "Type D" school buses also comply with FMVSS 222. With a body installed upon a truck chassis and an engine mounted in the front, midsection, or to the rear. Type D or transit buses have GVWRs well over 25,000 pounds empty. They have up to 84 elementary school student seating capacity. The entrance door and driver's seat are forward of the front axle. Type D buses have the same 16" aisles and 15 rows or more of 39" wide seats in tandem. The only three point seat belt-shoulder harness on the bus is for the driver. Compartmentalization in these buses is also designed for all students without the use of any seat belts.

The FMVSS 222 or School Bus Passenger Seating and Crash Protection for School Buses is intended to reduce the number of deaths and occupant injuries resulting from collisions of school bus occupants against structures outside the bus. Compartmentalization alone, however, will be shown below as not protecting students from rear end collisions, lateral collisions and rollovers as well as non-collisions during sudden driving maneuvers such as hard braking, sharp turning, or students being airborne during bus rear end elevation over rough roadways. The conventional interpretation of FMVSS is that by means of compartmentalization large school buses interiors will provide occupant protection so that children are protected without the need to buckle up.¹

The argument FOR school bus compartmentalization for Type C and D buses is based on these data:

The larger buses have a considerably larger mass than the majority of vehicles that collide with them giving student occupants a life-saving advantage. Because of the Type C and D construction school buses have different crash force distribution during vehicle-to-vehicle collisions.

Type C or D buses have high ground clearance with Delta V forces at the point of impact, or the point of maximum intrusion, occurring under the bus floor while the other vehicle underrides the passenger compartment. Bus passengers are thereby spared the highest crash pulse and the best ride down conditions compared to the others in the opposing vehicle.

US Fatality data since 1988 reveals that there were 416,000 fatal crashes of which 1,265 were school bus related. From 1988 to 1997 there were 115 fatalities on Type C and D school buses. Of the school bus collision fatalities 64% or two-thirds were passengers in the other, smaller vehicle. 27% of the school bus fatalities occurred outside the school bus—usually in the danger zone or an area some 10 feet on both sides or to the front of the bus. 10% of the fatalities were school bus occupants where compartmentalization may have minimized other fatalities and/or reduced other bus passenger injuries. Of the 10% fatalities in the bus 2% were drivers and 8% were student passengers. Since school bus drivers are always required to wear their three-point lap/shoulder belts compartmentalization for school bus drivers does not apply.

The Most Harmful Events (MHEs). For those 115 school bus fatalities cited above the MHEs came from 29 crashes with other vehicles, 14 fatalities from a collision with fixed objects, 6 fatalities from school bus rollovers, and 66 MHEs from all other bus locations.

Types of Vehicles in School Bus Collisions. Fatalities in the two-vehicle crashes occurred with large trucks with equal or greater GWVRs than the Type C or D buses 89% of the time. Light Trucks accounted for 7% of those school bus fatalities with the remainder coming from school bus collisions with passenger cars and other vehicles.

Direction of School Bus Collisions. Heavy truck frontal collisions with school buses accounted for 83% of the MHEs, Right-side lateral collisions were 10%. Left side collisions were 5%. 2% were rear-ending collisions. If we can assume that the same locations of collisions with lighter, passengers vehicles will mostly be from the front of the bus, with much fewer lateral collisions from either side of the bus and only 2 out of 100 being struck from the rear end—what does compartmentalization do then to contribute to passenger safety under those conditions?

For Type Two Collisions, or collisions of occupants within vehicles, compartmentalization provides passive occupant protection with energy absorbing, well-padded high seat backs that are strong and closely spaced in rows within the bus.

"It is difficult, if not impossible, to develop ways to protect school bus occupants in catastrophic crashes, such as those involving trains and heavy trucks. The crash forces in those crashes are so great that any reasonable structural design cannot maintain the integrity of the vehicle, which is one critical component of occupant crash protection. (National Transportation Safety Board)"²

We may therefore conclude that in catastrophic collisions compartmentalization does not provide sufficient student passenger safety for some or all passengers. Therefore in all other less-than-catastrophic collisions how well does compartmentalization work for all student passengers as it was intended?

The National Transportation Safety Board (NTSB) investigated a school bus accident in Conasauga, Tennessee, in which a school bus was struck on the passenger side by a locomotive traveling 51 mph. The NTSB concluded that sidewall components, such as the overhead storage racks, seat frames, and sidewalls, were not designed to be energy-absorbing; These structures could cause injury to occupants by lateral movement during a side-impact collision.3 As a result, the NTSB issued Safety Recommendation H-01-40 to NHTSA:

"H-01-40. Develop and incorporate into the Federal Motor Vehicle Safety Standards performance standards for school buses that address passenger protection for sidewalls, sidewall components, and seatframes."⁴

We may further conclude that no such sidewall protection, school bus sidewall components or seat frames have yet been implemented that would protect students from any lateral collisions. Seat frame strengthening or reinforced seat frames have not been developed in the industry since this recommendation was made. Overhead storage racks are no longer manufactured in newer buses. There are many older buses that have storage racks that endanger student passengers during collisions.

In a school bus accident in Central Bridge, New York⁵ the NTSB found that seat cushions came loose during impact. The Board subsequently concluded that the school bus passengers, whether lap belt-restrained or unrestrained, may have sustained more severe injuries because the seat cushion bottoms were unlatched. The NTSB made Safety Recommendation H-00-29 to NHTSA:

H-00-29. Modify the Federal Motor Vehicle Safety Standards to include the requirement that school bus seat cushion bottoms be installed with fail-safe latching devices to ensure they remain in their installed position during impacts and rollovers. Because enhanced performance for sidewalls, sidewall components, seat frames, and seat cushion fixation are critical for occupant safety in many accident scenarios, the Safety Board urges speedy action on these recommendations.⁶

NHTSA enforcement of seat cushion bottom securement to the seat frame has been adopted by school bus manufacturers. However, this leaves many older buses in the fleet not meeting that safety standard. When seat cushions become loose during bus lateral collisions or rollovers then compartmentalization is compromised. Without a seat cushion under them those students have no other safety measure to protect them from injury during collisions. Loss of collision integrity of the seat back, the seat cushion and/or the seat frame means compartmentalization cannot work as intended during all rollovers or many lateral collisions.

Does compartmentalization compromise passenger driver safety during school bus collisions? When would compartmentalization work or not work to save student lives? If compartmentalization cannot save passengers lives in catastrophic collisions, and the data in this article cites only fatalities, then how many more major or minor student injuries would compartmentalization prevent or not prevent? Heinrich's Pyramid⁷ predicted the ratio of fatalities to major and minor injuries in industry in general. Heinrich placed fatalities at the top of his pyramid. Mollenhauer⁸ in 1998 modified Heinrich's Pyramid to represent fatalities and lesser injuries with respect to collisions for both cars and trucks. The Mollenhauer truck injury data is a modified Heinrich Pyramid. The data is comparable to Type C and D school buses which are constructed of heavy truck chassis.(See Figure 1).

School industry leaders and the school bus industry as a whole tend to view compartmentalization as a safety standard fitting all collisions, all students, students in various positions in or out of their seats, students standing, students facing to the rear and other positions and not with their feet flat, facing forward with hands on lap.

The Compartmentalization Envelope (CE) is an envelope of protection--not a safety attitude--for school bus manufacturers, transportation supervisors, directors, and bus operators. Here is what the Compartmentalization Envelope (CE) literally means:

The CE is a cubic volume of space starting at the top of the seatback where the student is seated with a plane formed from to the top of students seatback to the next seatback in the row immediately in front. The end of the student's seat cushion near the bus wall is one of the side planes of the CE extending from the top of the student's seatback down to that student's seat base and then extending plane forward to the seatback side and cushion base of the row immediately in front. At the other end of the student's seat cushion adjacent to the bus aisle is the other side of the (CE) plane that also extends from the top of the student's seatback to the his seat cushion base and then extending plane forward to the seatback side and cushion base of the row immediately in front. Visualize the base of the CE cubic space as being level plane formed by the underside of the student's seat cushion (not the bus floor) and extending forward at that level to the seat base of the seat immediately in front. The student's 39" wide seatback is the back plane of the volume and the seatback immediately in front is the front plane of the CE cubic area. Multiple CEs are formed between each pair of adjacent seats in the school bus.

Further, because seatmates sharing the same CE may collide with one another during a collision the CE should be for one student passenger only so that there would be no other students with which to collide. If other students are present then it is problematic whether the CE can be effectively shared with seatmates. Lastly, any student secured with any lap belt or harness is by definition not protected within the CE since compartmentalization was defined according to NHTSA as not requiring students to use seatbelts.(See NHTSA citation #1 above.)

Within the compartmentalization envelope (CE) what size of students are best protected by the high-backed, well padded seats at their back and the seatback facing ahead?

The problem of Child Safety Seats. Smaller children with shorter legs that do not extend over the seat cushion and reach down below and outside the base of the compartmentalization envelope have the best protection. However, infants and small children under 40 pounds bodyweight are required to be secured on the school bus using Child Safety Restraint Systems (CSRS).⁹

Children using CSRS are not protected by compartmentalization because they are secured by a seat belt through the CSRS belt channel. The belt must be firmly secured to the seat frame. The Compartmentalization Envelope (CE) does not apply to any students using Safety Vests that require a cam strap securement behind them and around the back of their own seats. Children seated in booster seats on specially designed bus seats are secured with a three-way harness that is anchored to a reinforced bus seat frame. Booster seats with built-in harnesses are not afforded CE protection because they are secured with a seat belt.

Lateral Collisions and the Student's CE. Lateral collisions with students of any size will eject them from their CE and into the aisle or across the aisle against their fellow students who are seated in the row opposite them when moving toward the point of impact. Some students during lateral collisions are ejected from their CE and slammed against the opposite interior bus wall.

Rear End Bus Collisions and the Student's CE. During a rear end bus collision those students whose heads do not extend above their seatbacks will derive a benefit from their seatback acting as an effective back and neck restraint to reasonably assure their safe ride down. Older elementary students, or any middle and high school students are vulnerable to neck injuries from neck extension backward over their own seatbacks. In a high intensity rear end collision some students may ramp up and out of their seats backwards toward the point of impact. Students with neck extensions may also be those who are ramped backward and have exited their CEs. They, too, were not adequately protected from injury.

Frontal Bus Collisions and Their CE. When student heads are higher than the height of both seat backs—the seatback in front and the one on which they are seated—during a frontal collision those larger students are likely to suffer neck injuries suffering from neck flexion forward and over the seatback in front of them. Larger students in frontal collisions tend to ramp up and over the seat while moving toward the point of impact. When students ramp up in either direction (forward or backward) their CE is not working.

Students Not Properly Seated, Facing Forward and Their CE. Any students not seated facing forward with bodies turned toward the aisle, toward their bus wall, or facing rearward against the direction of bus movement, or kneeling in the seat facing forward or to the rear, or standing and facing in any direction while the bus is moving or other improper seating positions—all these students are outside their CE. For those ejected only from their seats but who remain within their seat bus row all other surfaces surrounding them are not padded surfaces and will not protect students colliding with bus walls, floors, seat frames, seat legs, the space under the seat in front or the space under their own seat. Those fortunate enough to remain within their CE may collide with their fellow seatmates, with backpacks, band instruments, lunch boxes, or books not secured to cause injury to those students in the collision mix.

Thirteen rows of seats should each provide a CE for student passenger safety during a collision right? What about the following rows or passenger placements on the bus?

Students with Front Row CE. Passenger side front rows face the stairwell stanchion with little or no padding on the frame and a metal modesty cover for the student seated there. The driver side front row seated students fare no better with the same type of stanchion in front of them and forward to the back of the driver's padded seat—none of which affords a modicum of padding as do all other padded seat backs in the seat rows behind. During frontal collisions students may be ramped up or ejected into the stanchion opening into the driver or on the passenger side row pummeled down into the stairwell. Damaged front exit doors allow their ejection from the bus. (Conclusion: Students in Front Rows Have NO CE.)

Students with Back Row CE. Passengers seated in the rearmost seat rows have the rear exit door and windows behind them. During a rear-end collision there are no students to ramp over onto them from the rear. However, student in rows forward from them may be ramped back into them. Students seated in the last row may be ejected from the bus through the rear aisle windows or from a rear door when that door integrity is compromised. Further, during that same rear end collision the students seated in the last rows receive the greatest amount of intrusion, are at or nearest the point of impact and experience the largest Delta Vs. (Conclusion: Students in Back Rows Have No CE.)

CE for Students in Seat Rows Distant From the Bus Center of Gravity. Students seated in rows progressively further away from the bus center of gravity (CG) are subjected to increasing G forces. During a collision causing the bus to yaw, to slide sideways or to spin without a rollover those students seated in rows most distant from the bus CG are ejected across the aisle to the adjacent seat row or are pressed against the bus wall on their side of the bus with other students ejected into them from the opposite row. (Conclusion: On either side of the bus aisle those students are outside of their protective CE or some of those students suffer an intrusion by others into their CE.)

Students in Seat Rows When Airborne Have No CE. Students are the most vulnerable to lateral displacement outside their CE during sharp turns and bus off-tracking at modest speeds. Onboard bus videotapes reveal that when the bus hits a dip or a speed bump or "curb hops" even at speeds under 10 MPH students will become airborne. (Conclusion: When airborne and off their seat cushions those students are outside or above their CE.)

CEs and School Bus Passenger RSIs. When buses are driven at normal speeds, and with no collisions, students in the last rows are jostled hard and continuously. The level of jostling is comparable to a continuous low-G bus collision for them. Adult bus attendants assigned to the back of the bus experience continuous jostling and have legitimately complained of back and neck Repetitive Stress Injuries (RSIs). Research on passenger seat displacement on buses using accelerometer data is critically needed. Passengers may receive RSIs when seated in the back of the bus when subjected to lateral forces that remove them from their CE while still remaining in their seat rows. (Conclusion: Minor up to major Repetitive Stress Injuries (RSIs) can and do occur because of repeated passenger displacement off their bus seats.)

CEs and Bus Passenger Seat Cushion Displacement. Higher Delta V bus collisions have caused some passengers to experience seat cushion failures as they return to their seating space after being airborne and out of their CE.¹⁰ Some students are pinned down within the seat frame after the seat cushion was ejected from that same seat frame.10 Other students may be injured by ejected seat cushions that become projectiles inside the bus or inside the seat row during ride down. (Conclusion: Students with seat cushion failures are extended below or above their CE. Ejected seat cushions can be a danger to all students on a bus in a collision.)

CEs for Passengers In Wheelchairs Does Not Apply. Lift bus lateral movements over rough roads and sharp turns all combine to physically challenge the already physically challenged. Injuries abound with students using manual or power wheelchairs when they are "jostled," tipped sideways, or ejected from their wheelchair if improperly secured.5 Wheelchairs users do not have a CE since their chairs are secured with four straps connecting the equipment with bus floor tracking and the student is secured with occupant restraint with a lap and shoulder belt system in place. (Conclusion: Students Using Wheelchairs Do Not Have Their CE.)

CE for Wheelchair Users Who Transfer to a Bus Seat. Wheelchair users who transfer to a school bus seat nearby may not be able to maintain their seated position on the bus without a three-way lap and shoulder belt built into a special, highly reinforced school bus seat. With no lap belt or a vest secured with a cam strap to their seat backs some of these students will slide out of their seats onto the floor during normal braking or especially when the bus is involved in a minor collision. (Conclusion: Students Using Wheelchairs When Transferred To A Seat May Not Have CE without added belt securements. Yet with belt securement students no longer receive a CE).

Students in School Bus Rollovers are Protected by CE? During school bus rollover collisions the CE for some occupants does not protect them. Some students receive compound collisions to their bodies when ejected from their seating area. Other students receive ejected students colliding against them and the bus wall. The first collision sequence is the crash pulse may occur in 1/10th of a second as the students onboard farther away from the point of impact are ejected from their seat toward passengers seated in rows nearest the point of impact. Students are slammed directly into their neighbors in the opposite row as they move toward the lateral point of impact or are pressed hard against the wall nearest the point of impact. The entire nation witnessed this instant process when ABCNEWS.com aired on November 6, 2006 on the World News a bus 900 rollover to the bus driver side.¹¹ When a compound rollover occurs the combined weight of students ejected across the aisle shifts the bus center of gravity. This sudden student bodyweight shift combines with the energy of the striking vehicle for the bus to rollover. The second collision sequence occurs when all students fall together "down" to the bus ceiling. The bus ceiling is not a passenger friendly surface. Like the walls, the floor and other bus surfaces these surfaces cause injuries. Students may fall six to seven feet from their seat rows against the ceiling head-, feet- or side-first during rollovers and outside their CEs. If the bus point of rest is upside down there remains the third collision sequence that occurs when students who are conscious and ambulatory attempt to evacuate an upside down bus. Slips, trips and falls and collisions with other students can and does cause further injuries. Should the bus rollover further to 1800 a fourth sequence would occur with students falling from the ceiling to the other side of the bus. Additional quarter-turn rollovers would put students in harms way as though they were clothing tumbling in a clothes dryer. (Conclusion: In all Rollover Collision all Rows are ejected from their Compartmentalization Envelope (CE) on one side of the bus. Wheelchairs users remain upside down and well secured with occupant restraints and wheelchair tiedowns in the bus that has rolled over.)

Students in School Bus Lateral Collisions are Protected by CE? In lateral collisions students opposite the collision side will be ejected from their CE in any direction that is toward the point of impact. Students on the collision side during a lateral collision will be pinned against each other and against the bus wall nearest that point of impact. At the same time students from the row across are ejected toward their fellow passengers toward the point of impact. (Conclusion: In all lateral collisions compartmentalization does not work for those students unless they are secured by a seat belt or are secured in their wheelchairs. Any person secured by a seat belt, by a three-way seat belt and shoulder harness in or out of a wheelchair may not receive protection from their Comparmentalization Envelope.)

CE as a passive student restraint system is effective only during a narrow range of collision types and intensities. Compartmentalization is not an all pervasive safety feature that exists throughout the school bus. Compartmentalization as a safety envelope requires students to be properly seated but fails to keep students safe when they are ejected out of that narrow space during a variety of collisions. Front and back bus rows, lateral collisions, rollovers, any student in a Child Safety Restraint System, a student who wears a vest anchored to the seat back—all these students may not have a CE that is required for their protection.

1. http://www-nrd.nhtsa.dot.gov/vrtc/cw/BUS_GIJune2000a.pdf dowloaded on 2/21/2007.
2. National Association of State Directors of Pupil Transportation Services, History of School Bus Safety—Why Are School Buses Built As They Are? (The Plains, Virginia: NASDPTS, 2000.
3. National Transportation Safety Board, CollisionofCSXTFreightTrainandMurrayCountySchool District School Bus at Railroad/Highway Grade Crossing, Conasauga, Tennessee, March 28, 2000, Highway Accident Report NTSB/HAR-01/03 (Washington, DC: NTSB, 2001.
4. National Transportation Safety Board.
5. National Transportation Safety Board, SchoolBusandDumpTruckCollision,CentralBridge,New York,October 21, 1999, Highway Accident Report NTSB/HAR-00/02 (Washington, DC: NTSB, 2000).
6. NTSB, August 6, 2002.
7. Heinrich, H.W., Peterson, D., and Roos, N. (1980). Industrial Accident Prevention. New York: McGraw Hill.
8. Mollenhauer, M. (1998). Proactive Driving Safety Evaluation: An Evaluation of an Automated Traveler Information System and Investigation of Hazard Analysis Data. Unpublished Doctoral Dissertation. University of Iowa, Iowa City.
9. NHTSA Child Safety and Restraint System accessed 2/22/2007 http://www.nhtsa.dot.gov/people/injury/buses/busseatbelt/page01.htm.
10. National Transportation Safety Board, Highway Accident Report, School Bus Run-off-Bridge Accident-Omaha, Nebraska, October 13, 2001. NTSB/HAR-04/01 (Washington, DC: NTSB, 2004).
11. NTSB Omaha, NE. Pg. 14.
12. ABC World News. November 6, 2006.