Enough is Enough, Part 4: Lapbelts and Compartmentalization

As we learned from the Introduction to this series, tire manufacturers, a motorcoach manufacturer and a seating manufacturer were sucked into a lawsuit in which they did not belong because of a combination of errors in an NTSB Report about the accident, and the shamefully-dishonest efforts of a handful of the victims’ attorneys who, among other things, accused two of these manufacturers for not equipping a motorcoach with three-point seatbelts – a motorcoach originally constructed at a time when virtually no motorcoach in the entire country contained them, and only two years after they were mandated, for new motorcoaches, in Europe. To better understand both the benefits and downsides of three-point seatbelts, one must first understand the array of problems that two-point seatbelts (i.e., lap belts) present.   

Further, one cannot really understand the genuine benefits and downsides of either two- or three-point seatbelts without also understanding a concept known as compartmentalization – the concept used for the seats on school buses of every size and mass, and which renders both two- and three-point seat belt systems of marginal importance in large vehicles, and profoundly dangerous in all school buses because of their tight seat spacing. Small school buses, and other small vehicles, certainly possess these problems. But these downsides are worth accepting with vehicles of smaller mass because the risks from ejection and rebounding in crashes involving them are even worse. But the same is not true for large buses and motorcoaches since, because of their far greater mass and the exponentially smaller crash pulses they experience as they swat away the small things with which they usually collide, the downsides of seatbelts are less acceptable, and in both my opinion and the opinion of NHTSA, not acceptable at all.

This installment will introduce the major principles involved in exploring the pros and cons of lapbelt installation, with occasional references to similar effects were three-point seatbelts involved. And it will also provide an overview of compartmentalization, and explain how the presence of this seating approach significantly reduces the need for any type of seatbelt, and in some cases (particularly with lapbelts), conflicts with it. Finally a few examples will be cited to illustrate these trade-offs. Part 5 of this series to follow next month will cite many more examples of these trade-offs, and help place the pros and cons of lapbelts, in particular, in a much broader, and hopefully enlightened and realistic perspective. Part 6 will explore these principles and trade-offs as they apply to three-point seatbelts.

Compartmentalization and Containment

The concept of containment – which a few years ago even some motorcoach manufacturers actually thought was identical to compartmentalization – is really quite different, and by comparison to compartmentalized seats, clearly inferior in most respects. While defined far less scientifically, “containment” is simply a collection of seating system features that helps to keep the passengers in their seats during moments when inertial and/or centrifugal forces would otherwise toss them out of them – including ejecting them through the vehicle’s windows or propelling them elsewhere within the vehicle, a phenomenon referred to as “rebounding.”



For a typical, pre-2009 motorcoach seat, its containment features include (a) a high seatback, (b) arm rests (at least when deployed in the armrest position), (c) contoured “bucket” seats (a feature that positions the passenger’s buttocks deeper into the seat cushion while providing two or three inches of seat cushion material outside and around that passenger’s hips) and (d) thickly-padded seat cushions and seat backs (compared to the far-less-impact-absorbing plastic seats commonly found on transit buses). While these features, combined, will not prevent ejections or rebounding in every accident scenario (including the rare rollover), they would certainly limit them in many accident scenarios – although not nearly as effectively as compartmentalized seats would (particularly if they contained all or some of the same features that conventional motorcoach seats already do).

An important shortcoming of mere containment is the fact that, other than the fact that it is there, and padded, the rear of the seatback contains little or nothing to protect the passenger in the seat behind it if and when he or she strikes the rear of this seatback. In fact, many motorcoach seatbacks have been rendered even more dangerous by the inclusion of a plastic fold-down tray embedded into the rear surface of the seatback. These characteristics are critical to seating safety because what most makes a seat safe is the seatback in front of it – particularly if that seatback’s safety is optimized, as it is in a compartmentalized seat.

In contrast to mere containment, compartmentalization optimizes the ability of the seatbelt to sequentially decelerate the various parts of a seated passenger’s body as it strikes the seatback in front from the rear. In an un-belted, free-flowing response to a frontal crash or other rapid deceleration, the last part of the body to strike the seatback in front is the passenger’s head. With compartmentalization, the acceleration of the passenger’s head into the seatback in front has been slowed down — by precise degrees of padding in selected portions of the seatback’s rear surface — several times before the head collides with the top or upper rear surface of the seatback. These sections of padding successively cushion each part of the body that strikes them. Within this sequence, the passenger’s knees strike the seatback first, then the shoulder, chest or another part of the torso, and finally the head or neck – depending partly on the size of the passenger, the height of the seatback, and of course the g-forces generated by the collision or “stopping short” of the vehicle. Within this sequence, the padding in the lower part of the seat back (where the passenger’s knees strike it first) is relatively soft (to slow down the acceleration of  successive body parts), the upper middle portion is padded a bit differently, and finally the top section is made even softer – to cushion the passenger’s already-slowed-down head when it eventually strikes the top of the seatback or the upper part of its rear surface. The density of foam at various portions of the seatback were carefully established and refined through an exhaustive amount of testing conducted by NHTSA, its contractors, and other agencies and companies, including some outside the United States.

One study, in particular, that illustrated the importance of this feature was a 1984 study by Transport Canada, in which three school buses were crashed into a wall at 30 mph (creating the 20-g-force collision – a level of force that serves as the benchmark for everything in public transportation vehicles from seat anchorage strength to the strength of wheelchair tie-downs and lap and shoulder belts). Because school buses are, as a structural matter, largely body-on-chassis “conversions,” where the mass of the chassis frame members is 30 to 50 times that of the body frame members (often mere “C”-channels, and not even tubular structures), the bodies of all three buses tested by Transport Canada separated from and shot forward (some by several feet) from the chassis, and the heads of the “anthropomorphic test dummies” (or ATD’s) lap-belted in place collided with the seatbacks in front of them at a force considered lethal, verily whipping them into the seatbacks at a greatly-accelerated force.

In contrast those dummies not seat-belted in place in these crash tests all “survived,” since the compartmentalized seats progressively lessened the acceleration of their heads into the seatback. As a consequence, the “head impact criteria” values (referred to, in industry jargon, as HIC values) of the unbelted ATDs never reached the value of 1.0 – the benchmark used to indicate that this collision would have been lethal to a real live passenger (whose head and torso were represented, in these tests, by ATDs designed to simulate the experiences of live passengers). 

One can see from this analysis how lapbelts and compartmentalization do not remotely operate together. In contrast, lapbelts not only render the benefits of compartmentalization worthless (except for those not using the belts), but worsen the damage a belted passenger would suffer striking any seatback in front, even one that is “compartmentalized.” Lap belts are clearly dangerous. As noted, these problems are acceptable on smaller vehicles because the lap belts at least prevent total ejections and greatly limit rebounding. Because automobiles typically collide with vehicles of the same or similar mass, ejections and rebounding are common, and obviously have severe consequences. So the damage done by lapbelts is considered a justifiable trade-off in vehicles of such mass compared to the more-certain death that passengers would suffer if ejected. But with buses or coaches that weight 10 times more than most vehicles with which they typically collide, and the inversely-geometrical relationship of their crash forces (for example a 2000 vehicle crashing into a 1000-lb. vehicle creates a crash-pulse ratio of one-to-seven for the larger vehicle), their vastly superior mass of a full-size bus or motorcoach would normally just “swat away” the lighter vehicle which with it would most often collide. As a consequence of these dynamics, the risks from the lap belts is considered to be significantly greater, in these large vehicles, than the risks from being ejected, while the opposite is true for vehicles of considerably lesser mass.

It is also critical to recognize that, with compartmentalization, the passenger need do absolutely nothing but remain in his or her seat to receive it’s benefits. This is why compartmentalization is referred to as a “passive restraint system.” In comparison, seat belts offer protection only if the passengers use them. And studies of usage by passengers of all ages (most studies have been conducted on schoolchildren) lie all over the board. Usage of school bus seatbelts is relatively high in the one State (New Jersey) largely because their usage – not merely their installation — is mandated and highly-enforced. But one early study of school bus riders in Houston found that only seven percent of them used their lap belts. So seatbelt usage, particularly where the seats are already compartmentalized – is an important factor in the consideration of installing lap belts or any belts, for that matter – since the added weight and cost is significant both as a manufacturing and purchasing matter and as an operating matter (more about this latter), and because of the many drawbacks of seatbelts – although three-point belts mitigate the worst drawback of lapbelts (the “fulcrum effect”) for most passengers in vehicles where the seats are not too closely-spaced together.



Obviously, seat-spacing has an impact on these dynamics. To minimize the rate of the head’s acceleration into the seatback in front (and also to crowd as many passengers into the bus as possible), school bus seat spacing is very tight – typically 24 to 25 inches (a distance actually set by State rather than Federal standards). And, of course, all these seats and seatbacks are compartmentalized. With seatbelts, the space between seats that keeps the belted passenger’s head from striking the seatback in front is known as the “envelope of restraint” – the distance which seatbacks much be spaced apart so as  to prevent a seat-belted passenger’s head (with either lap belts or three-point belts including a shoulder harness) from striking the seatback in front. For unbelted passengers, this tight envelope of restraint is of considerable benefit: Along with the seatbacks’ compartmentalization, the rate of acceleration of a passenger’s head into the seatbelt in front is greatly diminished.  

Of critical importance to the commercial reality of this phenomenon, at least two studies I have reviewed have concluded that, in order to safely eliminate the “fulcrum effect” from lap-belting passengers in place, seat-spacing would have to lie between 31 to 35 inches in order to protect the largest lap-belted passengers from striking the seatbacks in front. Complying with this spacing for stationary, non-reclining motorcoach seats would likely reduce the passenger capacity by 25 to 35 percent. But with reclining seatbacks that move the seatbacks roughly another seven or so inches further back – in a scenario where the seatback in front is reclined while the seatback behind it is not – the vehicle’s passenger capacity would need to be reduced by 30 to 40 percent!

Three-point seatbelts address this problem to a great degree, because they decrease this “envelope of restraint” considerably. But not completely – particularly on vehicles, like motorcoaches, which contain reclining seats. In a motorcoach with typical seat-spacing (not much larger than that of school buses), a tall “95-percentile” passenger even secured by a three-point belt system, and seated in an un-reclined seat behind a passenger whose seat is reclined, will smash his face, forehead or teeth into the top edge of the reclined seatback in front, and at an awesome, if not lethal, force if the vehicle strikes a relatively heavy object in front of it – like a fellow coach or truck. And while I know of no study that has measured this, the percentage of passengers vulnerable to this phenomenon may extend significantly beyond a 95-percentile-size passenger, thereby placing a far greater percentage of motorcoach passengers at risk, particularly if the vehicle’s seatbacks are not compartmentalized, yet to a considerable degree even if they are.

Regardless, this “fulcrum effect” is just the most significant problem with lap belts. There are countless others beyond the mere loss of seating capacity. This installment will conclude with a handful of scenarios that illustrate these problems – mostly related to the vehicle structure. Part 5, to follow next month, will cite a more broad variety of problems, including trade-offs that affect both capital and operating costs, as well as those that would translate into significant reductions in passenger capacity. Among the handful of exampled noted in this installment:



  • On body-on-chassis vehicles, like school buses and many “over-the-road motorcoach “conversions” like those built on “frame rail” chassis almost universally employed in truck construction (slightly modified for bus or coach bodies), if the seats are attached to anchorages just below the floor (and thus part of the bus or coach body), while the seatbelts are attached through the floor to anchorages connected to the chassis, when the bus bodies separate from the chassis — as they did in not only the 1984 Transport Canada tests, but in school bus accidents in Palm Springs (1991) and Fox River Grove (1995) — the lap belts would operate pretty much like old-fashioned hard-boiled-egg slicers, literally cutting the passengers’ bodies in half at the waist. In fairness, three-point seatbelts would not likely have as severe an impact, as the impact forces would be distributed among more parts of the body, particularly if the straps were wide. But in high-impact-force collisions, they too could slice-and-dice many belted passengers to some degree, and severely crush numerous body parts and internal organs of the others.


  • Regardless of the type of vehicle construction (body-on-chassis versus integral), the passengers’ internal organs would be compressed or crushed to some extent as the seatbelts engaged. Most vulnerable to these crash forces would be the not-yet-fully-developed organs of young children, and those of elderly and many disabled individuals, and the fetuses of women in the later stages of pregnancy: The more severe the collision or impact forces, the more severe the damage would be. Here again, three-point belts diminish the damage somewhat, by distributing the “loads” among more surface area of the body. At the same time, while the addition of the shoulder belt greatly reduces the “fulcrum effect” and decreases the “envelope of restraint” (permitting the vehicle to enjoy greater safe seating capacity), the addition of a shoulder belt to the mix creates additional problems, including neck injuries, crushing fetuses during the latter stages of a mother’s pregnancy, re-breaking broken arms in slings, and even strangling the passenger – depending on a range of factors aside of the passenger’s size, age and/or condition, such as the collision or crash orientation (oblique collisions and rollovers dramatically increase the potential for damage caused by shoulder belts, even as they largely eliminate the risks of total ejection and great reduce injuries from rebounding).


  •  When passengers are not belted into their seats, they fly off them the moment inertial and/or centrifugal forces are exerted upon their bodies, greatly limiting the forward “pull” on the now-bare seats because the weight of the seats is minimal (a typical school bus bench seat weighs about 70 lbs., a pair of deluxe motorcoach seats a bit more). But once you belt two passengers to that seat, its load easily quadruples or quintuples. So to keep the seat itself from ripping out of the floor, one must quadruple or quintuple the strength of the seat’s anchorages, as well as reinforce the seat cushion and seatback so that it is not deformed by such loads. This increased strength adds considerable cost to the vehicle as a purchasing matter. But the added weight also adds to its operating costs for two or more entire decades of usage, likely translating into tens of thousands of dollars in additional fuel and maintenance costs, including increased tire wear and replacement.



  • Frankly, if motorcoaches were weighed on truck scales, like they used to be, fully-loaded coaches (including a reasonable volume of luggage) equipped with seatbelts would often exceed the vehicles’ GVWR, and either a complete row or two of seats would have to be removed, or the rear suspension system would have to be “beefed up” substantially – converting an otherwise 8-tire vehicle to a 10-tire vehicle, with every other suspension system element enhanced as well, and likely requiring one grade higher of drive axle and its related components – and, of course, it’s added weight. As a consequence, the inclusion of seatbelts triggers a spiral of additional weight and cost penalties that could only be addressed by limiting the number of passengers (and their luggage) that the vehicle could transport.  

As noted, Part 5 of this series will overview a broad array of additional trade-offs which the installation and/or usage of lapbelts will effect. After absorbing these fundamental but critical principles which should be seriously considered in any decision – personal, commercial or institutional – to mandate or choose the installation of seatbelts (much less any regulations, industry standards or rules requiring their usage), it will be far easier to understand the trade-offs involved in the installation and usage of three-point belts. Not to be forgotten, of course, is the fact that while the addition of a shoulder harness to the seatbelt configuration mitigates many problems of lapbelts without them, the lapbelts remain a component of three-point seatbelts, and while somewhat differently, their continued inclusion in the now-expanded seatbelt configuration involves the same problems and trade-offs that lap belts do by themselves.

Publications: National Bus Trader.