Buying Tomorrow’s Buses Today, Part 5: Seating
Ned Einstein (Transportation Alternatives, New York, NY)
Anil V. Khadilkar, Ph. D (AVK Engineering, Huntington Beach, CA)
Like the pupil transportation community before us, the motorcoach community is hearing the clamor for seat belts. Citing European and Australian requirements for motorcoach seatbelts, these cries have been amplified, more recently, by a handful of catastrophic motorcoach accidents where seats have been torn from their anchorages or seating systems otherwise compromised.
Unlike school bus riders, motorcoach passengers enjoy some degree of lateral containment, at least when their arm rests are deployed. And they enjoy padded, often-contoured, forward-facing, high-back seats. Otherwise, with genuinely-compartmentalized seats, the pupil transportation community has been able to fend off the momentum for seat belts on large school buses for decades – with the exception, until recently, of only New York and New Jersey. Yet school bus seating systems provide no protection from lateral impact forces, nor even the containment provided by arm rests. Following its completion of a four-year study, even NHTSA was recently forced to acknowledge that compartmentalization was “incomplete.” Since then, five more states have enacted legislation mandating occupant restraint systems on all sizes of school buses. Some of these, like California, mandated three-point belts. Others, like Florida, mandated only lapbelts – which New York and New Jersey continue to require.
Catastrophes and Crystal Balls
In the lawsuit of one recent, catastrophic motorcoach incident involving seating system failure, the Media and Press focused largely on the seatbelt issue. However, another central issue was that the vehicle’s seats were not genuinely compartmentalized. One lesson from this landmark lawsuit is that, without compartmentalized seating, the absence of occupant restraints is glaring – particularly where the vehicle does not roll or tip over, and compartmentalization, by itself, might have mitigated the prevented the injuries, or at least lessened their severity.
This installment of NBT will summarize several noteworthy safety characteristics of existing motorcoach seating systems. More importantly, it will explore modifications that can be made to improve their safety and crashworthiness, in many accident scenarios, with or without the inclusion of occupant restraint systems.
Seating System Principles and Practices
The fundamental principle employed in the design of school bus seating systems is known as “compartmentalization.” The triplicate goals of compartmentalization are: (a) to contain the passengers within the seating compartment, (b) to provide this containment “passively” (i.e., the passengers need do nothing but remain in their seats), and (c) to dissipate the magnitude of crash forces to which they will likely be exposed in a collision. These goals are “managed” primarily by three system characteristics:
- All seats face forward
- All seats are uniformly spaced reasonably close together
- Seatbacks are designed and constructed to absorb the forces of the passengers’ forward movement
Lest the reader forget, this series of NBT installments is titled “Buying Tomorrow’s Buses Today.” So it is noteworthy that one seating manufacturer actually developed a high-back, genuinely-compartmentalized, motorcoach “activity seat,” and certified it to FMVSS school bus requirements (FMVSS #222, in 49 CRF 571.222), almost 15 years ago. Otherwise, motorcoach seats are not genuinely compartmentalized at all. This is true primarily because they do not satisfy the third requirement noted above: Optimal absorption of forces from the passengers’ forward movement.
When a vehicle stops abruptly (particularly in a front-impact collision, where g-forces are often high), crash pulses are transferred through the vehicle structure to the passengers, where they are absorbed by this structure, internal elements (e.g., seating systems) and, finally, the passengers’ bodies. During this process, three sets of phenomena occur in a sequence of milliseconds:
- The bus strikes the object.
- The passengers’ bodies thrust forward into the first interior object (or objects) with which they come in contact.
- Ligaments, axons and other “connectors” attaching internal organs to the skeleton jerk forward, rotate and/or tear, causing damage not always reparable – even when the internal organs to which they are attached remain relatively intact.
The best way to address the first of these phenomena is, of course, to execute accident-avoidance maneuvers. The second phenomenon is mitigated to some degree when the object struck is a relatively close-by seat back – assuming the seat’s anchorages can accommodate the “loads” exerted upon them. The third phenomenon is mitigated to a large degree when that seatback is designed to progressively absorb these forces or loads. In simple terms, much of what makes a seat safe is the presence, positioning and characteristics of the seatback directly in front of it, and the way it contains the passenger in the compartment.
In examining the benefits of compartmentalization, it is important to acknowledge that the three most common severe accident scenarios are front-end collisions, rollovers and tip-overs. Compartmentalization does not address ejections in rollovers and tip-overs – although in certain crash scenarios, compartmentalized seating can indeed impede some passengers’ exit paths. And, in truth, no technology is likely going to protect passengers positioned in the crush zones of excessive-force collisions. But compartmentalization and occupant restraints will prevent ejection and rebounding elsewhere in the vehicle. And compartmentalization alone will provide significant passenger protection in the majority of accident scenarios – particularly because compartmentalization is a “passive” technology: To receive the benefits of compartmentalized seating, all the passengers need do is remain seated.
Bearing the Loads
In genuine compartmentalized seating, the seatbacks are designed to sequentially and progressively absorb the loads created as passengers strike them. Because a passenger’s knee is closer to the seatback in front, it generally strikes the seatback first, and at a lower speed. The seatback then absorbs the knee impact, with the knee operating as a fulcrum to propel the torso and head forward. Because the passenger’s head begins this flight further away from the seatback than the knee, the head accelerates to a higher rate of speed by the time it reaches the seatback. Consequently, a central goal of compartmentalized seating is to reduce or “manage” the acceleration of the upper torso, chest and head following the knee impact.
In understanding compartmentalization, it is important to recognize that all passengers are obviously not the same height, weight or shape. Nor are their skeletal and muscular systems uniform in mass or strength. Both crash forces and crash orientations also vary considerably, and the crash forces that reach the passengers can vary geometrically. With such variation, the knee impact zone must effectively be designed to absorb a range of forces from a variety of angles. Achieving this goal is not a simple task. At the same time, existing motorcoach seats contain a number of features that provide an excellent starting point for further development.
Interestingly, even the form of compartmentalization designed for school buses (FMVSS #222) is not anathema to motorcoaches: While school bus seat-spacing is generally tight to maximize vehicle capacity, the maximum allowable spacing for even this form of compartmentalization is not very different than typical motorcoach seat-spacing. Nevertheless, optimizing compartmentalization will involve trade-offs among seat design, construction, materials and spacing.
Lap and Shoulder Belts
Close seat-spacing is not compatible with lap belts because this form of restraints transforms the passenger’s waist into a fulcrum that accelerates the movement of the head into the seatback in front. Using lapbelts, therefore, seats must be spaced reasonably far apart. Restrained in this fashion, passengers will not reach, and their heads will not strike, the seat backs directly in front of them. However, wide seat-spacing itself provides no containment, and thus the passengers must rely entirely on the occupant restraints for protection. Further, lap-belt-only restraints have their own limitations in moderate- to severe-frontal impacts: Jack-knifing (one’s knees and chin can strike one another) and/or submarining (passengers can slide beneath and out from under the belts). Three-point occupant restraint systems address all these problems: Three-point belts restrain a greater expanse of the passengers’ bodies, preventing jackknifing and submarining. And their smaller “envelopes of restraint” are compatible with the more closely-spaced seats which, if genuinely compartmentalized, would further “contain” the passengers in the seating compartment.
Lap belts conflict with compartmentalized seating in other, more complex ways. As an example, compartmentalized seating is designed for the knee, torso and head to strike the seatback in a certain sequence and direction, at certain forces. Lapbelts disrupt these design goals – changing the seatback positions at which various body parts strike corresponding portions of the seatbacks, and increasing the forces at which they do so. In simple terms, lap-belted passengers striking compartmentalized seatbacks can literally break their necks.
In comparing lap belts to three-point occupant restraint systems, it must be recognized that either system, properly designed and used, will prevent total ejection. However, in rollovers and tip-overs, lap belts may not prevent partial ejection (i.e., passengers’ heads breaking through the glass or passing through an open portal).
Both lap belts and three-point occupant restraints will inhibit rebounding. When the passenger strikes the seatback in front, much of the energy is absorbed by the deformation of the seatback. However, passengers bounce back from this initial impact. Occupant restraints keep them from flying about the passenger compartment in response to this bounce, and other bounces – particularly in rollovers, where passengers experience a sequence of varying impacts and changes in direction as the accident unfolds.
The value of occupant restraint systems in motorcoaches must also factor in the inherent problems of window retention and glazing – the former a natural bi-product of large window size. Largely because they recognized that compartmentalization alone would not provide adequate protection from rollovers and tip-overs, the European Union and Australia focused their regulatory changes primarily on occupant restraints. Such technology also acknowledge the high-speed environments in which motorcoaches often operate.
Because most modern motorcoach passengers are familiar with occupant restraints from their automobile-passenger experiences, they are more likely to use occupant restraints if they are (a) designed to encourage this usage, (b) integrated into the seat structure, and (c) similar to those of the passenger car environment to which they are generally accustomed. Regardless, with the existence of three-point occupant restraint technology, there is no reason to consider lap belts as a securement alternative.
Visions of Superiority
In discussing improvements to motorcoach seating systems, it is critical to recognize that the characteristics of existing motorcoach seating systems provide an excellent starting point for further development. And it is also critical to recognize that these improvements would enhance passenger safety in the majority of accident scenarios even without the installation and use of occupant restraint systems. Regardless, these safety characteristics have not yet been optimized. Nor have they been integrated into a truly-compartmentalized seating system. Nonetheless, these existing elements include:
- Arm Rests and Lateral Containment. The primary argument supporting the installation of seat belts on large school buses has been that their bench seats provide neither containment nor protection from lateral impact forces. This shortcoming is particularly problematic in school buses since they typically contain leaf-spring suspension systems (see “Buying Tomorrow’s Buses Today, Part 2: Structures and Suspension Systems” in NBT, April, 2006). In contrast, motorcoaches (and even many over-the-road buses deployed in motorcoach service) contain both pneumatic suspension systems and arm rests.
- High Seat Backs. In studies of school bus seating system safety, crash-testing efforts have identified seatback height as a significant variable. So it is important to recognize that\C2 most motorcoaches already contain high-back seats.
- Contoured Bucket Seats. While there are exceptions (e.g., older models deployed in commuter/express service), most motorcoaches contain contoured, individual-occupant, “bucket” seats. The contouring constrains lateral movement only minimally. However, when aisle-side passengers’ arm rests are deployed, the additive effect of contoured seating contributes further to their containment.
- Head Rests. U-shaped upper seatback head rests were developed largely to accommodate sleeping passengers whose heads might otherwise rotate or slide down – minimizing the stationary positioning conducive to sleep. However, these head rests may also limit the head’s rotation in a side- or oblique-impact collision scenario, or the severe swerving often necessitated by the driver’s execution of accident-avoidance maneuvers.
Additions and Refinements
By adding a few features, and tweaking many that already exist, a genuinely-compartmentalized seating system could evolve from current motorcoach seating configurations. Further, this starting point already contains many core elements of a better approach to compartmentalization than school buses currently possess. Features of a compartmentalized motorcoach seating system might involve a number of elements:
- Obtrusions. Fold-down “trays” need not be eliminated. Their structure and materials could simply change – including being collapsible. For practical motorcoach travel purposes, the platforms need only be strong enough to support the objects resting upon them – including heavy passengers leaning on their elbows. Otherwise, objects protruding from the rear of the seatbacks (magazine rack fasteners, cup holders, etc.) could become collapsible or cushioned.
- Footrests. In their current format, footrests conflict with compartmentalization – disrupting the normal movement of passengers into the seatbacks in front, among other problems. However, they could be designed to collapse, push away or break away. They could be made of materials other than steel that would still provide ample footrest support and stability. And they could also be padded.
- Head Rests. To inhibit lateral movement consistent with reasonable comfort, the depth of head rests could be increased, and their shapes and materials refined. One might also consider increasing their height, and changing the size, shape and padding of their rear profiles, in order to cushion forward-flying passengers propelled in slightly upward trajectories by bumps, dips or other variations of roadway configuration and crash scenarios.
- Support Structures. Like school bus seats, the design and construction of seatback structures and materials could be refined to decelerate the movement of passengers’ bodies and heads thrusting forward, and to progressively absorb impact forces. These characteristics must conform to the range of seat-spacing intervals appropriate for motorcoach passengers. The quick-change, track seating systems recently emerging in response to requirements for wheelchair accessibility, and the seat-to-seat variation created by reclining seatbacks, introduce considerable complexity into the design challenge. At the same time, it may be possible to actually increase seating capacity by constructing thinner seatbacks.
- Arm Rest Barriers. Existing “hard” and typically linear (i.e., bar- or tube-type) arm rests could be replaced with softer, contoured, full-profile panels. Apart from a trigger to retract them while passengers climb into and out of the seats, they could be designed to “default” to the deployed position – providing not only containment, but lateral impact protection. Such protection could prove invaluable in some multi-phase collisions where the post-impact trajectory of the vehicle causes passengers to change position within the passenger compartment. The structural characteristics could be graduated so that the upper surface operates as an armrest. And the supports could be designed to flex upon the exertion of significant forces, providing further absorption capabilities.
- Center-Seat Partitions. Like the partitions between the driver and shotgun seats of many automobiles and commercial aircraft, a fold-down partition could also be installed between the aisle- and wall-side bucket seats. However, to avoid the complexity and costs associated with air bags, these devices should not deploy automatically. However, if creatively designed, they would likely be used. If properly designed (e.g., with flexing and force-absorbing characteristics), their deployment could lessen the loads on the aisle-side arm rest in a collision (particularly a rollover) where lateral forces might otherwise pitch both passengers toward the aisle.
- Glazing. While ejection and window retention are somewhat endemic to window size, improvements in glazing offer the promise of improved passenger containment – especially if coordinated with improvements in the seating system.
- Modesty Panels. School buses contain seat backs – identical to those of the seats themselves – forward of front-row seats. These panels reflect the importance of seat backs to the compartmentalization equation. Depending on their spacing forward of the seats behind them, and other needs (e.g., security concerns for drivers), modesty panels for motorcoaches need not be identical to the seatbacks. At the same time, they should meet the floor surface, so that passengers’ feet do not get stuck below the barriers. As an alternative or (even better) a supplement, the front seats could be equipped with three-point occupant restraints – as they already are on many modern motorcoaches.
None of these features or refinements would preclude the installation of occupant restraint systems – preferably three-point restraint systems. Further, without some form of occupant restraint, there is no way to attach a child seat – exposing motorcoach manufacturers and operators to considerable risk in carrying young, small passengers with or without child seats. Regardless, occupant restraint systems could be “integrated” into the passenger-side surface of the seatbacks so that they would not protrude from the seatbacks when not in use. Such an approach might be more costly than free-swinging occupant restraint systems. However, they would eliminate or minimize many of the problems endemic to free-hanging lap and shoulder belts.
Perspectives and Choice
When one considers rollover and tipovers, yet also recognizes the fact that every passenger will not necessarily use passenger restraints even where available and easily accessible, it is clear that the safest approach to motorcoach seating would include both compartmentalized seating and occupant restraints. However, either of these technologies by itself would provide worthwhile enhancements to passenger safety. Compartmentalized seating systems – particularly those with the characteristics identified above optimized through focused design and development efforts – would provide significant benefits to motorcoach passengers in most accident scenarios. Further, depending on the crash orientation, they would impede the passenger’s path out the window in many rollovers and tipovers.
From a marketing perspective, an integrated occupant restraint system would accommodate the full range of passenger preferences: Those who would prefer to use seat belts, and those who would prefer not to. More importantly, with both technologies, we would not only be able to boast having the public transportation industry’s safest vehicles, but also the industry’s safest seats. At the same time, the enormous accommodations such devices would present as a safety matter would not require any trade-offs in passenger comfort.
From a liability perspective, any trade-off of safety for comfort is likely to translate into a finding of liability, and possibly the assessment of punitive damages. But the seating system envisioned above would not involve any such trade-offs between these variables. Instead, it would simultaneously enhance both safety and comfort.
Finally, from a political perspective, recent events in Europe, Australia and at least seven U.S. states demonstrate an inescapable reality: If we do not effect some improvements on our own, others will surely dictate them to us. Even more important to recognize, the dictators may define the details. Such dictators may take the form of state legislators, regulatory agencies, personal injury attorneys and even insurance carriers. Regardless, this variation would be far more disruptive to the motorcoach industry than it was to the school bus industry: This is simply because motorcoach service is largely an interstate phenomenon. If a handful of strategically-positioned states were to mandate occupant restraint systems, they could effectively dictate the “default characteristics” of most of the nation’s collective fleet.
Leapfrogging in Safety and Marketing
Motorcoaches containing a seating system like that described above would also win the smoldering debate about the comparative safety of providing school-related activity trips on school buses versus motorcoaches. While such trips represent only four percent of all school-related trips, they represent a whopping 30 percent of all motorcoach trips. Proponents of school buses providing such trips typically cite faux safety statistics – evidence mis-weighted because school buses are genuinely safer only for trips involving crossing, and because most serious school bus-related incidents do not appear in accident databases. However, these proponents also cite the superiority of school bus seating systems and window retention. Installing a seating system like that described above would dispel the former argument. Adding occupant restraints to the equation would vanquish the latter.
The failure to take proactive development efforts like those described above would also squander the opportunities created by this past year’s increase in the demand for motorcoach service, and the huge jump in vehicle sales that accompanied it. While this demand may continue as a bi-product of increased fuel costs, a reversal of this trend could dilute demand, reducing the volume-to-capacity ratio of our large vehicles and eroding the profits derivable from their deployment. Because motorcoaches are costly, long-term investments, they must be supported throughout their lives by measures to sustain ridership. And these measures must sustain enough profits to offset the increasing trends of liability exposure that have blossomed to balance our society’s shallow safety net and lack of a national healthcare program. Purchasing vehicles less likely to become obsolete as a safety and liability matter is an important component of our industry’s own healthcare program. Along with other elements identified in “Buying Tomorrow’s Buses Today,” the development and installation of a genuinely-compartmentalized seating system would both expand the industry’s potential market and limit its exposure.
Offense, Defense and Change
It is a popular sports cliché that the best defense is a good offense. In the closely-intertwined cauldron that safety and liability have become, this dictum is applicable to product development and product improvement. A dramatic reversal of these dynamics will not materialize from evolutionary refinement. It will materialize only from revolutionary change. However, as noted, the starting point for this revolution already exists in many current motorcoach elements. We need merely color it in, and draw a few lines outside the box.
A compartmentalized seating system, by itself, will not mitigate the risks of every accident scenario. But it will enhance passenger safety in most of them. More importantly, it will provide benefits to passengers unwilling to use occupant restraints even when they are available. Particularly when one factors in liability, such an improvement would appear to be both prudent and cost-effective.
The form of compartmentalization described above clearly goes beyond the format currently employed in school buses. At the same time, it is worth noting that school bus seats cost a fraction as much as most motorcoach seats. So, in the development of compartmentalized motorcoach seating systems, there is plenty of room to maneuver. There are, of course, other challenges. As noted, a formidable one is the fact that motorcoach seats typically recline. So effecting “compartmentalization” the instant before it is needed will involve solutions at both the design and operating levels.
As one can see, both compartmentalized seating systems and occupant restrains have a range of benefits and shortcomings. But their individual shortcomings are largely mitigated, their benefits enhanced, and the range of passengers protected more inclusive, when both technologies are employed, and both are properly designed and integrated. Under these conditions, both are better than either. But either is clearly better than none.