A report entitled “Transit Bus Research – Interim Report” provided details of research sponsored by Transport Canada, in affiliation with PMG Technologies Inc., involving testing of the crashworthiness of transit buses. This endeavor was initiated in response to recommendations made by Canada’s Transportation Safety Board (TSB) from its investigation of a 2013 collision involving a double-decker bus that collided with a passenger train near Ottawa, Ontario. The TSB commented that “a more robust front structure and crash energy management design might have reduced the damage to the bus and prevented the loss of a protective shell for the occupants”. As stated in the Transport Canada report, in 2015, the TSB issued the recommendation that “the Department of Transport develop and implement crashworthiness standards for commercial passenger buses to reduce the risk of injury”.
In response, Transport Canada commenced a programme “designed to examine the effects of structural stiffness and energy management on the protection of transit bus drivers and passengers during frontal crashes“. As part of this research Transport Canada purchased two, decommissioned, New Flyer buses from the City of Ottawa. Two full-scale crash tests were conducted in the orientation shown in the figure (taken from the report) below.
The tests were described in the report as follows:
“In each crash test, the driver side of a moving bus (Striking Vehicle) impacted the right rear corner of a stationary bus (Target Vehicle).
The impact speed of the striking vehicle was 40 km/h and the overlap at impact was 40 %, as shown in Figure 1.
In Test 1, which served as the baseline or reference test, both buses were in their original condition upon delivery. In
Test 2, the same vehicles were used, but the positions were reversed, i.e. the Target Vehicle in Test 1 became the new
This verbal description was also accompanied in the report by a schematic view of the test configuration as shown below.
A number of dummies were placed in the striking vehicle to explore what accelerations were experienced. A driver dummy was present as well as a number of dummies positioned throughout the bus interior as shown in the alpha-numerics noted in the above figure.
After the first test the structure of the striking bus in the vicinity of the bus driver was stiffened. Then the second test was run to see how the acceleration and structural intrusion were affected by this change. The stiffened areas are shown in the figure below, taken from the report.
The results of the 2 tests are shown in the figure below, also taken from the report.
Further graphing of the results was provided by comparing the differences in accelerations experienced by the dummies at their specific seating locations in the two tests. Again the figure below is taken from the report.
The authors of the report confirmed that “The stiffening, which was limited to a few structural members at the front of the bus, contributed to an average increase in the peak acceleration of approximately 1.8 g”. This is not a large increase, especially when the accelerations are examined at the driver’s position. A figure below shows the driver dummy in each test along with the resultant accelerations/forces as specific body parts of the driver dummy. Compared to the dummies in passenger seats the driver dummy sustained much higher accelerations (head = 166 and 54 g, pelvis = 30 and 24g).
A further figure, below, shows the final position of the dummy in each test as well as the extent of structural deformation around the dummy. There is obvious structural intrusion into the driver’s space in Test 1 but even Test 2 shows considerable interior damage.
What should have been obvious from these results is that there was a stark difference in injury potential between the driver seating position and those of the passengers to the rear. The authors of the report failed to make mention of that obvious fact.
However before discussing this further it must be acknowledged that the authors made some useful observations about injury potential to the bus passengers.
“Based on these preliminary results and the literature, a source of potential injury, on-board the bus appears to be interior
structures such as grab handles and seatbacks. Currently, there are no specific requirements designed to manage injury
risk associated with contact on these hard structures. By comparison, in passenger vehicles, several regulations and
technologies exist to reduce the force of contact with hard interior surfaces of the vehicle. C/FMVSS 201 “Occupant
protection in interior impact”, for example, prescribes a test protocol and defines injury criteria for head impacts with
instrument panels, and requires the use of energy-absorbing materials for sun visors and armrests.”
The lack of standards for improving energy-absorption during interior contacts on transit buses is an important observation. C/FMVSS 201 has been in existence for decades and has greatly improved the protection of occupants in passenger cars.
In their conclusions the authors suggested that there were some negative consequences from stiffening the bus structure. Three of the conclusions are noted below:
- Strengthening of the front bumper and front corner on the driver side reduced the intrusion into the driver
occupant space and resulted in significantly lower loads being directed to the head and legs of the driver.
- The strengthening contributed to an increase in the peak acceleration for the bus of approximately < 2 g.
- All dummies, with the exception of the driver, recorded generally higher responses in the strengthened bus.
While it is true that the the average acceleration at the passenger seats was increased by 1.8g, that increase is very small and would not increase the injury severity to the passengers to any large degree. Yet, even the stiffening of the bus structure around the bus driver still resulted in high accelerations (head 54g, pelvis, 24g) for the driver dummy.
What should have been obvious is that the cab-forward design of the transit buses provided very limited opportunity to protect the driver from significant injury. In a cab-forward design the bus driver sits at the very front of the bus, either on top of (cab-over) or in front of the engine and in front of the front suspension/wheels. There is very limited distance between the front bumper of the bus and the driver’s seat. And there is no meaningful bus structure in front of the driver. This means that in any frontal impact, due to the large mass of the bus, much of the energy dissipation must occur within the immediate vicinity of where the driver is seated. Even the most ambitious attempts to control that dissipation would be challenged with the introduction of structural intrusion into where driver is seated. And this is exactly what the Transport Canada research has shown in Test #1.
While it may be argued that this cab-forward design is needed in transit buses so that drivers have a better view of objects and pedestrians in front of the bus, that is not necessarily true. School buses also operate in similar conditions as transit buses and school buses have their engine and front wheels/suspension in front of the driver, as shown in the example photo below.
The inclusion of additional structure at the front of bus is crucial to improving the safety of the driver in higher-severity, frontal impacts. Once this additional structure is in place the dissipation of energy is a simpler problem as there is more time and distance available to achieve the result. And the proper re-design of transit buses incorporating a crushable front end over a longer time and distance will improve the levels of acceleration of the bus passengers to the rear.
It needs to be emphasized that the tests described in the Transport Canada sponsored research were not of a high severity. The striking bus was travelling at only 40 km/h and it impacted a stationary bus of identical mass. While some important details of the tests were not provided in the report, in a real-world collision one might expect that the striking bus sustained a change-in-velocity of about 20 km/h. That is not large when considering the types of collisions that occur on many rural highways and expressways.
In fact one might ask, why would a transit bus with a cab-forward design be allowed to travel on any expressways? Clearly it has been seen on numerous occasions on expressways such as Highway 401 in southern Ontario that instances of heavy braking occur as stopped traffic is encountered and, quite frequently the front end of one vehicle impacts the rear of another, in a similar manner as the tests conducted in the current report. While many of these rear-end impacts involve contacts with a smaller car or light truck, it is known that many portions of Highway 401 contain heavy truck volumes of over 50%. It is only a matter of time before some of these transit buses with be involved in frontal impacts with such heavy vehicles, or immovable roadside structures, and thus the change-in-velocity of the bus could be at least 20 km/h or higher.
Still, the most important point, is that the Transport Canada research reviewed in this article does not address the problem that was identified in the TSB concerns. What was obvious in the 2013 impact between the bus and the train is that there was a predominant lateral force in the impact and this resulted in the bus being cut open as if by a can-opener. Conducting fore-aft, purely-frontal, impact tests does not replicate the type of impact force that occurred in the train-bus collision. What needs to be understood is why the bus structure disintegrated as it did, from a predominantly lateral impact force. Thus it is not the stiffening of the front structure in a purely frontal impact that needs to be applied. It is a question of attachment of all the body panels and structures of the bus so that, when a lateral impact occurs those panels and structures do not simply separate but they continue to be held together and thereby achieve the desired energy dissipation by their unified deformation/buckling/crush.
The tragic consequences of poor transit and coach bus structural integrity are frequently demonstrated in collisions where needless deaths have occurred. Many of these consequences remain unreported. In April, 2018 the infamous Humboldt Broncos bus crash demonstrates how little focus has been placed on the performance of bus structures even when numerous fatalities occur. In the Humboldt Broncos crash the on-site photos showed the bus lying on its side with with its roof completely separated from the remaining structure. Most observers believed that this separation must have occurred when emergency personnel cut the roof to gain access the occupants. Yet the cause of the roof separation has never been publicly determined. Similarly the complete front end of the bus appeared to be separated from the remaining structure but no discussion has ensued about why this occurred and how it related to the collision severity. No one has spoken of how the numerous fatalities were related to the crashworthiness of the bus structure.