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September 01, 1999  |   Comments (0)   |   Post a comment

Evaluating the Safety of Lap Belts on Small School Buses

By focusing the seat-belt debate on large school buses, have we overlooked the potential danger of lap belts on small buses?

by John Painter


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The Great Seat-Belt Debate continues in earnest. For many, that debate is limited to whether seat belts are needed on large school buses. Small school buses are already equipped with lap belts. Many of the people who oppose lap belts on large buses accept the need for these restraints on small buses, although their use is seldom mandated. Is there really a different principle at work in small buses than in large ones? Are the dangers of lap belts in small buses outweighed by the dangers of not having them? While attending the National Transportation Safety Board’s (NTSB) bus crashworthiness public hearing in Las Vegas last year, I heard this argument from a concerned parent: “If lap belts are so bad, then why do we have them in small school buses?” A good question.

NHTSA’s rationale appears sound
In 1977, the National Highway Traffic Safety Administration (NHTSA) mandated lap belts on school buses with a gross vehicle weight of 10,000 pounds or less. NHTSA’s rationale at the time appeared sound: Small buses are more like passenger cars in that they will likely experience greater deceleration forces in a collision than bigger, heavier buses. Therefore, occupants should be offered the same restraint benefits. We now understand that lap belts actually isolate the most fragile areas of our children’s bodies. In frontal collisions serious enough to benefit from restraints in general, lap belts may apply huge concentrated forces to those vulnerable areas. The soft abdomen and lower spine are compressed as the small child’s pelvis often cannot retain the lap belt and it slips upward with potentially hundreds of pounds of force pressing against the soft abdominal organs and fragile spinal cord. Meanwhile, the child’s disproportionately large head and frail neck take the brunt of deceleration forces as the upper torso pivots around the belt, and the head strikes the rigid seat frame violently flexing the upper spine. Similar injury can occur in violent side impacts as well.

Unexpected results
The lap-belt puzzle began to unfold years ago when investigators discovered that lap belts in the rear seats of passenger vehicles often caused more serious injuries than they prevented, even in fairly moderate collisions that resulted in little injury to lap- and shoulder-belted occupants in the front seat. A myriad of lawsuits against auto manufacturers, whose engineers warned them of the dangers of lap belts, have been settled for undisclosed terms. Aggrieved parents and children had at one time banded together to form a “lap-belt survivors” group to lobby for safer restraints. It is perhaps tragic irony that we now have parents and others who are banding together to put those same restraints into school buses.

Ejections prevented
What could be worse than lashing our children to the deck of a vehicle with a piece of flat rope? NHTSA says the only thing worse is the alarming regularity with which unrestrained small bodies may be thrown through relatively large and brittle car windows, most often in high-speed rollovers. If ejected onto concrete, passengers may suffer fatal injuries. Those thrown onto dirt or soft vegetation are more likely to survive. Yet partially or fully ejected occupants are often killed when their own vehicle rolls on top of them. Ejection prevention is truly the only clear attribute of lap belts. NHTSA crashworthiness and statistics experts have concluded that were we not subject to ejection-related injury, we would generally be safer with no belt than with a lap belt. Hence, lap/shoulder belts are now required for (almost) all positions in a passenger vehicle. The evidence against lap belts is clear. In passenger cars, even 20-mph frontal impacts into rigid objects have killed and crippled lap-belted children, while others who are unrestrained walk away unharmed. These deaths are not aberrations. Through military testing on live baboons, the fatal effects of lap belts have proven to be predictable over a range of deceleration forces. Lap-belted subjects — highly muscular baboons with large pelvises not likely to slide beneath the belt — died from peak accelerations ranging from a low of about 19 g’s (A “g” is equal to the gravitational force exerted on a body at rest.) to a high of about 57 g’s. But baboons in four-point restraints (lap belt with two shoulder belts, as used by pilots and race car drivers) were much better protected, suffering fatal injury only at much higher accelerations between 62 and 145 g’s. Three-point restraints were not tested, but the protection afforded by three-point belts such as those in passenger vehicles would fall between these extremes, and well above lap belts alone.

Lap-belt use urged
In 1983, the NTSB recommended, based upon a limited analysis, that lap-belt use be mandated in small buses. Few states complied. Perhaps the others were stopped short, because it was then that the evidence against lap belts began to explode. In 1984, Transport Canada (the Canadian equivalent of the U.S. Department of Transportation) performed 30-mph fixed-barrier crash tests indicating that lap-belted test dummies in both large and small buses suffered potentially life-threatening injury far exceeding that suffered by unrestrained occupants. Many of the dummies in the small buses might have died in a real-life collision. Compartmentalization, which is designed to spread deceleration forces across the entire torso of the occupant, performed far better in frontal collisions. In 1986, the NTSB released its own study of lap-belt performance in passenger vehicles, concluding that lap belts in the 26 cases studied performed very poorly. The threshold at which fatal lap-belt injury to children might occur was very low. The baboon test data mentioned earlier correlates well with the fatal injury threshold noted in the NTSB study. Lap-belted children died from lap belt–related injury in passenger vehicles suffering as little as a 20-mph “delta V,” or “change in velocity.” A typical peak acceleration for a passenger vehicle striking a rigid fixed barrier at 20 mph is about 18 g’s. Remember, sturdy baboons died in simulated crashes with peaks as low as 19 g’s. The NTSB rejected the idea of lap belts in school buses in its two-volume examination of more than 120 accidents involving both large and small school bus crashes, released in 1987 and 1989, respectively. Both reports concluded that more children may have been killed than saved had all been wearing belts. In one accident involving a school van (not a regulation bus), a child died from lap belt–induced injury in a 25- to 28-mph delta V collision. The human costs or benefits of lap belts in all school bus collisions could not be ascertained in the NTSB study, but there was no indication that overall safety would be enhanced. Other incidents, such as the tragic Palm Springs downhill runaway and collision with large boulders, prompted NTSB investigators to acclaim the effectiveness of compartmentalization, noting that only those in the direct impact area were killed. Then, in November 1993, the NTSB investigated a collision in Snyder, Okla. A 20-passenger, 8,600-pound school bus was struck broadside by a 66,000-pound truck. The truck was traveling at an estimated maximum speed of 65 mph and left short skid marks before impact. On the basis of this single investigation the NTSB recommended lap–belt installation and mandatory use in small buses. Four children seated in or near the impact area were killed. The board report concluded that some of those were killed from ejection, not primary impact. The NTSB dusted off the same lap-belt recommendation for small school buses made in 1983, prior to its own lap-belt research. The NTSB’s official position was that the 1983 recommendation had never been withdrawn.

Crash data is limited
What do we know about lap-belt performance in serious frontal collisions in school buses? We don’t know much. Why? There are several reasons. Serious frontal collisions are rare. School bus drivers usually travel at moderate speeds and pay attention to where they are going. They are unlikely to be speeding, seriously fatigued or intoxicated. As a result, they seldom run directly off the road into roadside hazards. Serious frontal collisions in which passengers are wearing lap belts are all but non-existent. Lap belts have been disdained by most school bus transportation professionals for some time. Even when provided, they are seldom used. Many of us who oppose lap belts have been reluctantly waiting for New Jersey or New York to provide us with a test case involving frontal delta V of 20 to 30 mph. Thankfully, none has yet occurred. Therefore, the debate continues. We would prefer that this type of data be collected in the laboratory or test ground, not in the field. The roadside hazards school buses strike are often incapable of withstanding the significant mass of the large bus, and shear or move from the bus’ path when they would otherwise have created fatal deceleration forces for the occupants of an automobile. In a collision with other road vehicles, the school bus usually wins. In a head-on collision between a 2,000-pound car and 20,000-pound school bus, both traveling at 30 mph, the car will suffer a change in velocity of about 55 mph, while the bus will slow only 5.5 mph, a 10 to 1 ratio. School bus drivers need fear primarily other heavy commercial vehicles and trains. However, those collisions with large vehicles are often side impacts, as would be expected. The larger and longer the bus, the greater the exposure to lateral impacts when crossing railroad tracks or busy roadways. And there is little that can be done to insulate those in the direct impact area from the forces of such a collision, but that is another subject. Real experience in school bus crashes does not mirror that of passenger vehicles. Heavy frontal decelerations in school buses are rare, and perhaps unlikely, whereas such frontal impacts account for more than 50 percent of all passenger vehicle fatalities. Certainly, serious frontal collisions can occur in school buses of any size. Some collisions capable of lap belt–induced fatalities may have occurred, but been disguised by the superior performance of compartmentalization. Successful performance does not draw attention.

Here is the argument
Lap belts are more dangerous in small buses than in large buses for the following reasons:

  • Small buses are more likely than large buses, simply as a function of their weight, to be involved in frontal crashes with other vehicles in which they sustain a 20 mph or greater change in velocity.
  • Changes in velocity in a collision between two vehicles will be inversely proportional to the vehicle weights. In other words, in a head-on collision between a 10,000-pound and a 20,000-pound vehicle, the lighter vehicle will suffer twice the change in velocity as the heavier vehicle.
  • Per the Transport Canada school bus tests in 1984, small buses may be proportionally stiffer than large buses, when total mass is considered, and will likely create higher g loads upon contact with fixed objects or other vehicles.
  • Because of the tendency of a heavily loaded large bus to sustain extensive crush, it may be very difficult to realize a frontal-impact accident scenario in which sufficient deceleration forces are created to result in fatal injury to lap-belted occupants. Not so for small buses.
  •  

    Here are my conclusions
    If every child in America today were firmly lap-belted into a school bus seat, it might be years before we suffer a lap belt–related fatality. And in the interim, we might save the lives of several children who would otherwise have been ejected from the fairly small bus windows. Conversely, tomorrow might bring a single 40-mph collision between a school bus and an unforgiving bridge abutment. Sixty lap-belted children could suffer fatal head or neck injuries, while perhaps 10 others who were derelict in their restraint usage could simply be treated and released. Only then would the public demand to know how such a tragedy could have occurred, given all the evidence. That is the gamble, and it is a real one. The debate about lap belts in school buses, large or small, boils down to the following: Lap belts could occasionally save some children from ejection-related or other injury. There will always be public examples of serious injury to students that would not have happened had a lap belt been worn. But those who oppose lap belts in school buses are not willing to gamble that a serious frontal collision will not occur and are not willing to accept the personal responsibility for a single event in which needless massive fatalities result from lap belt–induced injury. The performance of lap belts in side impacts is also a concern, as the same principles apply. In our zeal to find the answer for large buses, have we forgotten small buses? Have government recommendations for the use of lap belts in small buses been misguided? If there is no indication that occupants of small buses are more likely to suffer ejection-related injury than those in large buses, then what positive function do lap belts perform in these vehicles? Have bad science and bureaucratic inertia caused us to risk unnecessary injury and death? Small buses account for only a minor portion of the annual student transportation mileage, and passenger use of the belts provided is rare. Therefore, perhaps no lap-belt fatalities have been realized. But one issue is certain. Mile for mile, a lap-belted occupant of a small school bus is far more exposed to harm than is his counterpart in a large school bus, far more likely to encounter circumstances that will make that lap belt his enemy rather than his friend.

    John Painter is president of TARAS (Traffic Accident Reconstruction Animation and Simulation) in Arlington, Texas. Website: www.schoolbusaccident.com


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