Occupant Protection in Buses - A Brief Review of the Current Literature
By: Mark Cohen
For: Suzanne Tylko
Table of Contents
- Canada's Regulatory Environment
- Collision Types and Crashworthiness Research
- Chronological Review of Experimental Research Found in the Literature
As of 2011, over 1.1 million Canadians use public transit buses to commute to and from work Footnote 1. Coupled with the approximately 2 million Canadian children who travel by school bus every day, these 3.1 million commuters represent just under 10% of Canada's population Footnote 2. While this might seem like a relatively low number compared to the 12.27 million Canadians who commute as either drivers or passengers in personal motor vehicles, or to the 23.5 million children travelling on school buses daily in the United-States, it still represents a significant population of people at potential risk for vehicle collisions Footnote 1, Footnote 3.
On September 18th, 2013, a VIA Rail Canada Inc. (VIA) passenger train was involved in a collision with an OC Transpo double-decker transit bus at a railway crossing in the Smith Falls Subdivision of Ottawa, Ontario Footnote 4. The bus was travelling at approximately 8 km/hr with the brakes applied when it collided with the train, an event which resulted in the front of the bus being completely sheared off Footnote 4. The VIA crew/passengers were uninjured, while amongst the bus occupants there were six fatalities, nine serious injuries, and about 25 minor injuries. Five of the six fatally wounded passengers were ejected from the bus Footnote 4. The most common, non-fatal injuries included broken bones, lacerations, and contusions. According to the Transportation Safety Board (TSB), the mechanisms of injury were primarily due to passengers being ejected from the bus, falling out of their seats, falling from a standing position, being struck by another passenger, being struck by other items, or a combination of the above Footnote 4. Following an investigation, the TSB recommended that: “The Department of Transport develop and implement crashworthiness standards for commercial passenger buses to reduce the risk of injury” Footnote 4. In response to these recommendations, the Crashworthiness Division developed a three-year research plan with the objective of investigating ejection and injury mitigation countermeasures for unrestrained passengers of highway, commuter, and transit buses.
This literature review represents the first step in the Crashworthiness Division's plan, with the aim of setting up a framework to help guide studies on potential occupant injury countermeasures and injury mitigation techniques. This report presents a summary of relevant research into the crashworthiness and occupant protection of buses, with a focus on experimental analysis and testing.
Canada's Regulatory Environment
The current regulatory environment has different requirements for buses depending on their gross vehicle weight rating (GVWR) or intended use (e.g. transit bus vs. school bus). For example, in Canada, there are 18 Canadian Motor Vehicle Safety Standards (CMVSS) which apply to buses as a category, with three of them intended only for school buses. Buses with a GVWR above 4,536 kg are required to comply with only seven standards, while older buses or buses crossing the border into Canada are not required to meet any standards Footnote 5. While these applicable standards include areas such as crashworthiness of the body and frame, and interior occupant protection, the standards are, at their core, based on old data and have a limited scope of applicability depending on a bus' classification Footnote 6. Additionally, there are provincial and territorial level standards that have the potential to impact occupant safety, though these focus primarily on vehicle usage and not manufacturing Footnote 5.
In Canada, buses with a GVWR below 4,536 kg are required by CMVSS 208 Footnote 7 to have either Type 1 or Type 2 seatbelt assemblies; that is pelvic only restraints (Type 1) or a standard 3-point restraint (Type 2). Contrarily, buses with a GVWR above 4,536 kg, such as transit buses, are not required to have seat belts in any position save for that of the driver Footnote 5, Footnote 7. Current regulations do not require transit buses to have any forms of occupant protection for passengers, though manufacturers must comply with applicable federal regulations if they chose to install non-required equipment, such as seat belts Footnote 5, Footnote 7.
It should be noted that transit and coach bus manufacturers may choose to meet the American Public Transportation Association's (APTA) Technical Specifications if they so desire. These specifications, set out in the APTA Standard Bus Procurement Guidelines, contain requirements for heavy-duty transit buses and commuter coaches Footnote 8. These guidelines include technical specifications for structural integrity and contain requirements for front and rear impact protection at low speeds Footnote 8.
In comparison, school buses are also not required to have passenger restraints but are designed around the occupant protection notion of compartmentalization as set out in CMVSS 222 Footnote 9. This feature is one that provides passive occupant protection in frontal collisions by way of deformable and energy-absorbing seats, as well as optimized seat spacing and seat back height Footnote 10. In other words, the seats or barriers in front of occupants, save for the driver who is required to have a type 2 seat belt, act as energy absorbers in the event of a frontal collision.
Unlike in Canada, the US Federal Motor Vehicle Safety Standards (FMVSS) require school buses with a GVWR below 4,536 kg to have lap/shoulder belts at all seating positions Footnote 11. They also require type 2 belts at all seating locations in “over-the-road” buses (buses with an elevated passenger deck located above a luggage compartment) and any bus with a GVWR above 11,793 kg, except for transit buses, school buses, and perimeter seating buses Footnote 11, Footnote 12. There are amendments currently proposed by the Canadian Department of Transport to implement regulatory changes that would harmonize the Canadian Motor Vehicle Safety Regulations and the US FMVSS with respect to restraints in buses, but they have yet to be accepted and adopted Footnote 12.
Collision Types and Crashworthiness Research
There are four crash configurations that are of interest when discussing the crashworthiness of buses: frontal impact, side impact, rear impact, and rollover. While all of these collision types are destructive in their own ways and pose a risk of injury to vehicle occupants, their different methods of action call for different design considerations Footnote 6.
Matolcsy defines six subject categories related to frontal, side and rear impacts: safety bumper, strength of passenger seats and their anchorage, retention of passengers, driver's protection, integrity of important structural elements, and interior arrangement Footnote 13Footnote 14Footnote 15. The safety bumper acts as an elastically deforming barrier in very low speed collisions, and a semi-elastic to plastic deformable barrier at low to high speeds with the express goal of limiting deceleration through the use of a crush zone Footnote 13Footnote 14Footnote 15. Designs focus on limiting the deceleration while having easily replaceable parts. Passenger seats and their anchorages, as well as retention systems (seat-belts) act to anchor the occupant to a surface that allows for the dissipation and redistribution of loads. Therefore design considerations include the limiting of decelerations, forces, and deflections on the body Footnote 5, Footnote 13, Footnote 16. The protection of the driver is different than that of regular passengers. Due to the exposed position at the front of the vehicle, the driver's compartment must be designed to not only contain occupant protection systems such as airbags and seatbelts, but to limit any structural intrusions which can injure the driver Footnote 13. Lastly, structural elements must be engineered to allow the bus to deform in such ways as to reduce the risk of injury to the occupants, while the arrangement of the interior should aim to mitigate the potential for injury through the optimal positioning of structures such as poles, handles, and cushions Footnote 13.
Matolcsy defines three important focuses for rollover instances: having a required roof strength to assure a survival space for passengers, mitigating passenger ejection from their seats or the bus itself, and avoiding uncontrolled motion within the compartment Footnote 13Footnote 14Footnote 15.
Chronological Review of Experimental Research Found in the Literature
It was decided to search for and review publications that focus on laboratory and field testing, as opposed to numerical simulations or statistical analysis. The following section gives brief summaries of relevant publications in order of publication date.
Beginning with an investigation of the use of seat belts in intercity buses, this paper describes the resulting barrier impact test at 40 km/h with 26 adult and child dummies. Some of the seats detached due to both buckling of the floor and failure of the fasteners. The results of the initial test were used to guide a series of frontal sled tests with 10g deceleration, where the seat anchorages had been improved over the preceding one. LaBelle concludes that 10g decelerations are adequate to test seat systems Footnote 17.
Kruger in Footnote 16 discusses how the statistical vulnerability of the elderly and children are a focal point for increased efforts of the development of more crashworthy buses and coaches. Hypothesizing that bus seats have the potential to be used as restraints, the author performed sled tests at a speed of 24 km/h with row distances varying between 700-1000mm. After 18 tests with a pulse of 5g and time duration of 90ms, the responses of 50th percentile male and 6-year old child dummies were examined. The results showed that the head loads of child dummies and the overall risk of injury steadily decreased with increasing distance to the next seat front, while adult dummies showed a minimum risk of injury at a row distance of 800 mm. The author concludes that the optimal seat distance to favor both children and adults is in the range of 800-850 mm. Furthermore, the author notes the suitability of seats to be used as restraints due to their large shape, close contact with passengers, and sturdy anchorage to the vehicle body, while maintaining that passenger safety is determined by the energy-absorbing capacity and deformation characteristics of the seats.
A second series of tests were conducted and described in Footnote 16 to observe the response of seat anchorages under decelerative loads. The tests were frontal impacts with a peak deceleration of 10 g, and with the seats kept in the optimal range as concluded by the first series. In these tests, the original seat anchorages failed during impact; an event detected in real-life accidents involving both coach and double-decker buses. These failures were shown to severely decrease the effectiveness of the seats as occupant protection systems, while further tests with reinforced anchorages showed once again their suitability to reduce injury risk Footnote 16.
Dal Nevo et al., 1991
Inspired by the two most severe coach collisions in Australia's history, a coach to truck collision at 200 km/h with 19 dead, and a coach to coach collision at 200 km/h with 35 dead, the authors compared the findings from accident investigations of these and other collisions to the a series of tests performed according to the ECE 80 regulation Footnote 18. It was concluded that the ECE regulation's test method did not adequately represent real-world damage, and that passive 10 g protection would not offer sufficient protection. The authors concluded that a multi-faceted safety system is required, including the vehicle body having increased rollover strength, emergency exits, and Type 2 occupant restraints. A seat was developed incorporating a Type 2 harness which offered adequate protection to 20 g decelerations Footnote 18.
Dickison and Buckley, 1996
Dickison and Buckley Footnote 19 present a novel system which enables the fitting of replacement seats with integral belts into minibuses. At the time of publication, the majority of minibuses and coaches in use had either no restraint system or simple Type 1 pelvic restraints. Due to public demand for increased occupant safety, manufacturers were either retrofitting their seats with belts or developing new seats with integrated belts. Testing performed at the Motor Industry Research Association (MIRA) in England showed that bolting new belts onto existing seats was inadequate due to the seat anchorages and mountings being unable to withstand the increased applied load, causing the seat to detach from the floor or collapse onto the passenger. Furthermore, while occupant safety was increased due to the integration of seatbelts, the floor structure of the buses were not designed to withstand the newly increased loads, and failed during static load testing Footnote 19. The MIRA team developed an under-floor reinforcement system which enabled the fitting of replacement seats with integrated seatbelts into minibuses. The team concluded that conversion of pre-existing buses from a solely seat perspective is generally inadequate, and a thorough development program is necessary to ensure a proper reinforcement of the floor structure Footnote 19.
Berg and Niewohner, 1998
Berg and Niewohner performed three full scale bus crash tests, one rollover and two frontal, to observe the causes of injuries to occupants. The rollover test used two lap belt restrained and three unrestrained 50th percentile Hybrid III (HIII) dummies as passengers (all dummies were instrumented save for the third unbelted dummy). The bus was accelerated to 40 km/h using a traction cable, which was unhooked as it ran up onto a ramp, until it rolled over as it reached a sustained speed of 30 km/h. The authors reported that the measured injury criteria values of the dummies were significantly lower for the belted dummies compared to the unbelted, with the most significant differences being in the head injury criteria (HIC) and resultant head acceleration. It was concluded that seat belts prevent passengers from being thrown into the central area of the bus, which would reduce the incidence of severe injury Footnote 20.
Two frontal crash tests were performed, the first using one belted and one unbelted dummy, while the second used the same number and configuration as the rollover test. The first test was performed at 40 km/h with a 70% overlap into the rear of a stationary 16 ton truck with the brakes engaged, and the second was performed at 31 km/h with a 30% overlap with a rigid barrier. The authors concluded that though the driver and persons around him are at an increased risk due to potential structural intrusions into the area, the flexibility of the front structure of the bus leads to a relatively low level of deceleration in the passenger area, meaning that both belted and unbelted passengers have a relatively low risk of injury Footnote 20.
Mitsuishi et al., 2001
Mitsuishi et al. in Footnote 21 performed sled impact tests at speed of 25 and 35 km/h with three instrumented Hybrid III dummies in two rows. Two dummies were fastened with 2-point (Type 1) lap belts (one in each row) while the second front row dummy was left unbelted. The front row had a distance between a simulated bus service box of 360 mm and the second row had a distance from the front row of 860 mm. The authors' conclusions discussed the limitations of lap belts in preventing head impacts, and the potential of reducing occupant injury by varying the distance between rows Footnote 21.
Elias et al., 2001 and Hinch et al., 2002
Both papers present research done by the National Highway Traffic Safety Administration (NHTSA) to evaluate the potential of safety restraints on large school buses, by means of two full-scale dynamic crash tests (one frontal impact and one side impact) and a series of dynamic sled tests using restraint alternatives. Elias et al. published a preliminary paper Footnote 22 at the 18th International Technical Conference on the Enhanced Safety of Vehicles (ESV), while the work by Hinch et al. Footnote 6 was submitted as a report to the US congress at the conclusion of their research Footnote 6, Footnote 22.
The first impact consisted of a conventional Class C school bus impacting a rigid barrier at 48 km/h. Instrumented Hybrid III dummies were present in the vehicle, consisting of two 50th percentile adult males, two 5th percentile adult females, and two Hybrid III 6 Year Olds. The dummies were placed in upright seating positions as rearward as possible with respect to the seat cushion. Post impact observations showed a clear forward movement of the occupant cabin due to the standard bus construction of having the body mounted to the frame rails of the chassis by a series of clamps; a feature that, according to the authors, allows for a dissipation of impact energy over a longer period, resulting in decreased levels of acceleration. Head Injury Criterion (HIC15), Neck Injury Criterion (Nij), and chest acceleration values were calculated for all anthropomorphic test dummies (ATDs) Footnote 6, Footnote 22.
The side impact test consisted of an 11,406 kg cab-over truck impacting the side of a stationary Class D transit-style school bus at 72.4 km/h. The impact point was chosen such that the truck impacted just behind the front axle of the bus. Seven instrumented dummies were used: one Hybrid III 50th percentile male with a single tri-axial accelerometer in the head, two 5th percentile female HIIIs, two 6 year old HIIIs, and two 50th percentile male Hybrid III/SID dummies. Post impact observations showed that the truck penetrated approximately halfway into the body of the bus. HIC15, chest acceleration, and thoracic trauma index (TTI) were calculated for all ATDs Footnote 6, Footnote 22.
The sled tests were designed to assess occupant size, restraint strategies, loading conditions, seat spacing, and seat back height. Occupant sizes of interest were:
- An average six year old (represented by a Hybrid III 6 Year Old)
- An average 12 year old (represented by a Hybrid III 5th percentile female)
- A Large high school student (represented by a Hybrid III 50th percentile male)
Three different restraint strategies were assessed:
- Lap belt (coupled with compartmentalization)
- 3 point seat belts (on a modified seat with a non-FMVSS 222 compliant back)
Other factors of interest were:
- Seat spacing
- Seat-back height
- Rear occupant loading
The dummies were loaded in three different seating position and restraint combinations to evaluate different strategies and factors concurrently: restrained occupants without any loading from occupants behind them, restrained occupants with loading from unrestrained occupants seated behind them (compartmentalization), and unrestrained occupants into a seat back in front of them (compartmentalization). The dummies' kinematics and instrumentation data were analyzed and the following was concluded:
Low head injury values were observed for all dummy sizes save for when override occurred. High head injury values were present when the 50th percentile male dummy overrode the seat in front of it, while high-back seats prevented this. Approximately half of the child and 5th percentile dummy Nij values were high. Compartmentalization is sensitive to seat back height, does not appear to be sensitive to rear loading conditions and seems to be moderately influenced by seat spacing.
Lap Belt Restraint:
Type 1 belts effectively keep ATDs in their seats. HIC values were low for all dummies. Nij values were high for most dummies, and were generally higher than those from compartmentalization tests. Neck injury potential is very sensitive to seat spacing and occupant size, with a large number of tests producing Nij values more than twice the desired threshold.
Type 2 belts effectively keep ATDs in their seats and HIC values were low for all dummies. HIC was drastically lower than for the other two restraint types. When the restraints were properly positioned, Nij values were below 1.0 for all dummies. It was noted that there is a significant potential to produce undesirable outcomes if the restraints are improperly used.
By modifying the seat back to allow for the increased loading applied by the seatbelt, the back became stiffer and lowered the injury mitigation effect of compartmentalization. Stiffer designs could be overcome through reengineering seat designs or by modifying the requirements of FMVSS 222.
Transport Canada's Standards and Regulations Division performed a study to investigate the relative safety offered to children travelling in school buses by the ways of compartmentalization and child restraints. Tests were performed on an acceleration HyGe sled using different dummies to represent varying child anthropometry, with the test conditions complying with Transport Canada's Test Method 213 – Child Restraint Systems (delta-v of 48 km/h, 20 g deceleration pulse) Footnote 10. Two tests were conducted for an 18-month CRABI dummy using compartmentalization and a child restraint, two tests were performed for 3- and 6-year old Hybrid III dummies using compartmentalization only, and three tests were performed for the 3-year old dummy using child restraints. The results of the study allowed the team to conclude that children whose mass is 18 kg and under, or until they reach approximately 4.5 years old, would benefit from being secured with child restraints for their appropriate size, and that older children continue to be well protected by compartmentalization Footnote 10.
Tylko and Charlebois, 2010
In 2010, Transport Canada's Crashworthiness Division investigated child occupant protection in school buses by means of a crash test program that included full-scale frontal and side impact crash tests, coupled with sled tests to assess potential countermeasures Footnote 2.
A full frontal rigid barrier test was conducted with a 2006 Ford E350 Girardin Futura school bus being impacted by a rigid barrier at 48 km/h. Different sizes of instrumented dummies in different seating positions and using different occupant restraints were used to assess the level of injury for all respective combinations. The authors' analysis showed that the severity of the crash was beyond the protective limits of compartmentalization, and that raising the height of the half barrier at the front of the bus and the addition of energy attenuating padding should be considered to improve containment of the dummy occupants and potentially reduce head injury responses Footnote 2.
Two side impact tests were performed, one with a 2009 Thomas SAF-T-Liner school bus impacted by a moving rigid barrier, and the other with a 2006 Ford E350 Girardin Futura school bus impacted by a Dodge Ram 1500 pickup truck. In the first test, the bus was impacted by the barrier at a speed of 48 km/h. The resulting dummy responses from the head, neck, shoulder, chest, abdomen, and pelvis of 5th percentile female, 50th percentile male, 3-year-old, and 6-year-old side impact (SID) dummies were below their respective injury limits Footnote 2.
The second test had the stationary bus being impacted by the pickup truck at 60 km/h, a much more severe impact not only due to the speed, but due to the similarities in vehicle mass (4,355 kg vs. 2,461 kg). The resulting crash caused both the ejection of an adolescent sized dummy on impact, and the ejection of an adolescent sized dummy on rebound. The results showed that compartmentalization failed to contain the dummy occupants in their seats, and that an absence of energy absorbing padding, especially in proximity to the window frame, may have caused an increased risk of injury to the head. The authors concluded that existing measures of protection through the use of compartmentalization are not adequate for side impact collisions due to the current design of bus seats being unable to prevent ejection Footnote 2.
Further work was performed to investigate potential occupant protection countermeasures to address the observed limitations of compartmentalization and optimize school bus crash protection. This basis was that an optimal protection system must:
- Provide an overall reduction in the risk of injury for frontal, side, and rollover collisions;
- be appropriate for all age groups, sizes, and behaviours;
- be practical for all users; and
- be affordable.
The first investigated countermeasure was three-point (Type 2) seatbelts. Due to the increased loading caused by occupant restraints during an impact event, the authors compared normal bus seats with reinforced seats designed to handle the increased loads using the test method of CMVSS 222. The results of the comparison tests showed that both the regular and plywood reinforced seat produced HIC values well below the limit of 1000, yet had peak head accelerations that were more than double the 80g magnitude considered the acceptable limit, suggesting that the head injury criterion may not be an adequate measure of head injury risk in seats equipped with three-point occupant restraints Footnote 2.
Sled tests were then performed to further investigate three-point seatbelts, with the results showing that even in a low severity crash, unrestrained occupants seated in seats equipped with three-point seatbelts may be exposed to an increased risk of both head or leg injuries Footnote 2.
The second investigated countermeasure was lap belts (pelvic restraints). Sled tests with Hybrid III 6- and 10-year old and 5th percentile females seated in standard school bus seats modified to accept lap belts that could be adjusted to two different anchorage locations were performed. Results showed that most injury measures were below injury thresholds, though the head of the dummies had a tendency to rotate posteriorly when impacting the front seatback. An observation was made concerning the biofidelity of the Hybrid III 6-year old dummy; this being that the discontinuity between the flesh of the pelvis and upper legs, combined with the inflexible seated posture of the pelvis, confounds the recorded dummy injury measures because the upper body and head behaves like a pendulum Footnote 2.
Energy absorbing padding and inflatable curtain technologies were the last countermeasures investigated. Energy attenuating padding was found to effectively reduce head and chest injury measures for child and adolescent dummies, while inflatable curtains coupled with window frame padding significantly reduced the HIC values for adolescent sized dummies Footnote 2.
Li et al., 2013
Li et al. Footnote 23 investigates injuries in children to the neck, chest, and femur under different restraint conditions through a series of laboratory sled tests. Five tests were performed using two different seat bucks, and a selection of instrumented ATDs. Injury criteria for the above mentioned regions were calculated, and while head, chest, and femur results were within acceptable limits as set out in FMVSS 208, the neck injury thresholds were surpassed in tests where compartmentalization and lap belts were the only restraints used. The authors concluded the following Footnote 23:
- The neck is the most easily damaged region of the body in school bus frontal collisions using standard occupant restraints;
- Compartmentalization and lap belts are insufficient to protect the neck, though are acceptable for protecting the head, cheat, and femur;
- Improved seatbacks should be developed if only compartmentalization and lap-belts are to be used; and
- Three-point seatbelts offer suitable protection to all investigated body regions Footnote 23.
Though a topic of interest for over fifty years, investigations into suitable methods for improving occupant safety and injury risk mitigation in different types of buses remain scarce Footnote 5. Each bus type is subject to different federal standards, and therefore has variable levels and forms of protection. The current literature focuses primarily on seat-belts and compartmentalization, and lacks investigation into alternate injury countermeasures Footnote 2, Footnote 5, Footnote 6, Footnote 10, Footnote 16,Footnote 17,Footnote 18,Footnote 19,Footnote 20,Footnote 21,Footnote 22,Footnote 23. Further research is needed to properly design new or improved forms of occupant protection, with the goal of reducing the potential for occupant injury during not only frontal impacts, but during all crash configurations. It is also apparent that there is a distinct bias towards school buses in the literature, necessitating further investigation into occupant protection in other common bus types such as transit buses.