Police Push-Bar Advantage in RCMP Stevenson Crash Clarified

In  a May 3, 2020 article published by several CBC journalists about the head-on collision between Constable Stevenson and the mass murderer Gabriel Wortman in Nova Scotia, a substantially inaccurate quote from a road safety expert exaggerated the safety advantage of front bumper push bars on police vehicles.

The substance of the article dealt with the fact that Wortman’s replica police vehicle was equipped with a push bar and Stevenson’s vehicle was not so equipped. The inaccurate quote reportedly came from “a professor emeritus…who is a pedestrian advocate and road safety expert”.

The article indicated that, according to the expert “When their cars collided on the morning of April 19 on a Nova Scotia highway, the gunman would have had a ‘massive’ advantage”.

That comment could be allowed if there was sufficient information about this specific crash. But it is understood that little of the details are known. But further in the article came the following:

“The person in the vehicle that does not have the bars would feel, in effect, the equivalent of 100 per cent of the impact,” the expert reportedly told CBC News. “The person with the push bar, the amount of impact felt by the vehicle driver is minimal relative to the person who’s getting hit. Huge difference”.

Unfortunately the above statement is an obvious exaggeration. While the Professor may have been exaggerating to prove an important point, the extent of that exaggeration needs to be clarified.

Push bars do not, and cannot, prevent a vehicle from experiencing the force of an impact. Newton’s Third Law has clearly stated for centuries that “for every action there is an equal and opposite reaction”. This simply means that two collision partners in a head-on collision experience the same magnitude of impact force. Wortman may have sustained some advantage from his push-bar-equipped vehicle but that has been poorly explained.

We build crushable vehicles so that they absorb or dissipate the total kinetic energy of a head-on impact. Thus the structures of both vehicles have the job of dissipating that energy while also prolonging the length of time when this dissipation occurs.

An important observation can be made by examining the results of a hypothetical, very-stiff Sherman tank impacting a car in a head-on impact, with both vehicles travelling at 60 km/h. In such an impact, given the likely, very large, mass difference, the driver of the Sherman tank would experience a minimal change-in-velocity while the car could experience one almost equal to the closing speed of 120 km/h. And the soft structure of the car would be used up until there was massive intrusion into the car driver’s occupant compartment. Thus the death of the car driver would result from both, a much smaller mass, and due to the structural intrusion that would defeat modern safety features.

But if the Sherman tank then struck an immovable, concrete wall of a bridge at 60 km/h the opposite would occur. The much more massive bridge wall would stop all of the tank’s forward motion, and it might even push the tank backwards due to the elastic nature of the impact (i.e. where there would be a high co-efficient of restitution). The additional point is that, due to this high stiffness of both structures (the wall and the tank) the change-in-velocity would be almost instantaneous. It is this very short time interval of change-in-velocity which is crucial. The very short time of velocity change means that the acceleration experienced by the tank might also result in very high acceleration experienced by the driver of the tank so he would surely die.

Now an additional mind experiment. What if the tank had a very large volume and was soft? Even though the bridge wall was very stiff the tank’s large volume could be “crushed” over a long time and distance, the driver could experience low levels of acceleration and the driver could survive, even though the same change-in-velocity was experienced.

Now another mind experient. The soft tank with the large volume now stikes a police cruiser with a push bar on its front bumper. We assume that the push bar is properly attached to a stiff portion of the police cruiser equivalent to an old-school frame. In such a case the push bar makes the cruiser’s front end stiffer. When the impact occurs with the soft and voluminous Sherman tank we would expect a great deal of crush of the structures of both vehicles. But we do not know the exact extent of crush because that would be dependent on the ratio of stiffness between the two partners. So if the police cruiser was slightly stiffer it would crush less while there would be more crush in the structure of the soft tank. But the point is that both drivers could benefit from this crush, not just one driver. Remember Newton’s Third Law. It is the overall softness of the structures of both vehicles that are involved in the impact that decides what safety benefit can be potentially achieved by both drivers. So even though the driver of the car would experience a very large change-in-velocity due to the mass difference, the greater time and the greater distance over which the velocity change is experienced could be a benefit to that driver.

The quoted comment by the expert Professor is an obvious exaggeration in the present context as the driver of the car that does not have push bars would not experience “the equivalent of 100 per cent of the impact”. That is a regrettable error. The exaggeration was  likely made to emphasize that a benefit would be gained by the stiffer vehicle and, conversely, its driver.  If the stiffer vehicle sustained less crush there was less chance of defeating the benefits of air bags, seat-belts, etc by reducing the potential of structural intrusion into the driver’s seated space.

In the above text we purposely used words such as “could” and “potentially” with respect to injury causation for an important reason. There is a difference between the “severity” of the impact to the vehicles versus the “severity” of the impact experienced by the occupants. It is the safety features in each vehicle which decide how the collision force is applied to the occupant’s body. Thus thee driver of a vehicle equipped with modern air bags, seat-belts and other features will experience a much different force than the driver riding in a vintage car with none of those features. So the existence of any push bars would only be one factor amongst many in determining what would happen to the driver.

The important point that the expert Professor was making is that there are many vehicles on the road with very different structures. These differences have an effect on an occupant’s safety when an impact occurs. A police cruiser with a push bar gains an advantage over the same vehicle without one, given a certain type of head-on collision. However such advantages exist on a constant basis with all vehicles. We do not pay enough attention to reducing those differences when it is possible to do so.

We talk about our freedom to choose the vehicle we drive and its features. Thus we decide that we will purchase a large and tall, 4-wheel-drive, pick-up truck for example. We then lift its suspension. Yet perhaps all we needed was a high-payload work vehicle for our business use. In almost all severe, head-on impacts mass always wins. But, additionally, higher stiffness often wins. And if the structure of your vehicle is higher than the structure of the opposing vehicle then height also wins. But these wins only apply to the fact that you will kill someone and not yourself.  You have a choice whereby a collision might result in the survival of you and the other driver but you reject that choice. A very selfish act.

This is not much different than buying an automatic, rapid-fire, “AK-47” style weapon when a single-shot rifle might do for rural, farm purposes. This was the mindset of Gabriel Wortman.

It is our aggressive mindset that results in needless injury and death to others because we believe we have the right to do as we please.