A very low speed rollover of a school bus in north-west of Toronto yesterday could have been deadly – so what made it a successful non-event?

Was it pure luck, engineering, or a combination of both that led to minor consequences in this school bus rollover?

To begin with, there was no information about the specific location of the mishap. It reportedly occurred on Cold Springs Camp Road “Ganaraska Forest”. This remote area may have specific road conditions relevant to the cause of the collision. Although this is a hard top road surface many tar and chip surfaces contain substantial bumps and upheavals, particularly in the springtime and it would help to know what the road surface conditions were like.

Every person who has ridden in a school knows their childhood experiences whenever a school bus travelled over a bumpy road. In fact many adventurous boys would specifically sit in the back of the bus on these occasions because this would be the location where the bus would exhibit the largest vertical motion while traversing the bumps. While children find this entertaining there is a darker side to the issue.

School buses have stiff suspensions. When riding over bumps they exhibit large motions in their sprung mass. The sprung mass is that portion of the body that is riding on the suspension, or the mass minus the wheels, axles and suspension. That is why bus drivers and the companies that operate school buses need to pay attention to where the bus will be travelling. Especially in springtime when roads may change their character due to the warmth/frost cycle that causes road surface upheavals.

In documenting the road surface conditions of a particular section of roadway several years ago that was travelled by school buses I documented the conditions noted in the two photos below.

Example of a road surface upheaval on a roadway regularly travelled by school buses.

A crack in the pavement producing several inches difference in elevation can be a significant issue to the safety of a school bus.

These surface ruptures existed through several locations along this road. When documenting schools buses travelling over the bumps I was approached by a local police officer who interpreted that my documentations were a way of stalking the bus drivers who had observed me videotaping them and photographing the school buses on several occasions. When I pointed out to the officer that the road surface conditions were a danger to the safe operation of the school buses and that he should be reporting that danger to the municipality, he merely concluded that this would be of no use as the municipality would not listen to him. He also threatened that I discontinue my documentations or I would be charged.

So why is there such a difficulty in officially documenting the road surface conditions, especially when it may involve the safety of innocent children in a school bus? We know that these children will not be seat-belted and that the loss-of-control of a bus could result in a rollover. The kind of rollover that occurred at the Cold Springs Camp Road, but which could potentially result in much more serious consequences.

What can be said about the present collision is that the rollover of the school bus occurred at a very low speed. One can follow the tire marks on the sloped ditch as they terminate near the final rest position of the school bus. The tire marks end precisely in a lateral position next to the bus meaning that, just as the bus was coming to stop it fell over. So the forward speed of the bus was not the important matter. What was important, and potentially dangerous, was the low speed rollover.

This is where school bus design comes in. As an accident investigator under contract  to Transport Canada in the 1980s I and my fellow investigators were required to participate in a Special Projects program where there was an emphasis on documenting school bus collisions. At that time it was recognized that  there were many safety issues with school buses. As an example, sharp, sheet-metal panels installed in the interior body of the bus would separate even in minor collisions resulting in unnecessary lacerations, some of which could be serious. In other instances the seat backs if school buses contained metal tubes along their tops that resulted in unnecessary injury to a child that struck these stiff and narrow structures. Most importantly it was recognized that passenger ejection was a large threat due to the large windows and ease of ejection when rollover was a common mechanism in school bus crashes. There were many changes made to the design of school buses with introduction of “compartmentalization”, or the idea of keeping children confined to the local area where they were seated. Thus high-back seats were installed and the side windows were reinforced to create smaller openings to prevent ejection. And of course the sheet metal junctions were corrected.

The term “dice in a box” is commonly used to refer to the consequences of rollovers in that, one can engineer many things to reduce injury consequences, but no one can actually predict what will happen in an individual rollover as they are so unique. Yes, the general pattern of commencement of loss-of-control is common. But even then loss-of-control can occur, and often does, from an initial, minor impact with a lighter vehicle. Road surface condition is also a major and common factor in school bus loss-of-control. But once the loss-of-control begins the status of many factors comes into play as to what will happen next. What we see in the present case is that the school bus encountered a slope of the ditch next to the road and that was the primary factor in the rollover. But it was fortunate that there were no other objects present where the low speed rollover occurred. So large rocks could be danger. Trees and poles could also be a danger. And a factor that is less publicized is the presence of water.

Springtime and heightened water in ditches is not something that should be unexpected. Yet there is little appreciation of the fact that even shallow amounts of water in a ditch can be lethal to occupants of vehicles that overturn into such ditches. It was fortunate that the overturned school bus did not land in a water-filled ditch, that it was travelling slowly, that the bus was not fully loaded with passengers, and that the collision occurred in relatively warmer conditions and not in a winter blizzard or darkness.

When Transport Canada considered what needed to be done to improve the safety of school buses it was looking precisely at collisions such as this one where there would be a relatively low speed impact, or a loss-of-control, that would eventually lead to a rollover. What was crucially important is the prevention of ejection of the children and, fortunately, in this case, ejection was likely prevented. If a child had been ejected one can imagine that the full mass of the bus could fall onto the child and the consequences could be deadly. But one can never tell what will happen when you have “dice in box”. It is no different than a Monte Carlo casino where, in the vast majority of instances you win nothing or very little, but every so often you have the misfortune of hitting the deadly jackpot that you don’t want.