We use cookies on this site to enhance your online experience. By continuing to use this site, you agree to accept cookies.


Truck rollovers can be serious. They can result in property damage, personal injury, and even death.

Those truck drivers who survive a rollover face a good chance of being charged with dangerous driving. Upon conviction, that offence brings with it a mandatory driving prohibition of at least one year, as well as a fine (or maybe jail) and 10 penalty points. A conviction also has the effect of voiding all insurance coverage, and so the driver will be personally responsible for paying for the damage to the truck, as well as any claims made by any other motorists for property damage, injury, or death.

For those of you who think “I am a careful driver – it can never happen to me”, please read on. A Canadian Transport engineer has written “Rollover is not a gradual event and the driver is usually unaware there is a problem until it is too late. Most drivers operate very close to the rollover threshold and do not realize it – there is almost NO safety margin.”

What causes a truck to rollover? Simply put, that occurs when the forces trying to push the truck over (called lateral acceleration) exceed the forces trying to keep the truck upright. Obviously, though, there is more to it than that.

Every vehicle has what engineers call a rollover threshold. That is the maximum amount of lateral acceleration that a vehicle can tolerate without rolling over. A passenger car has an RT of around 1.3. A pickup truck has an RT of around 1.0. A loaded tractor and pole trailer has an RT of around .3. A loaded tractor and tri axle trailer has an RT of around .28.

A number of factors go into determining the rollover threshold. The two most important factors are the height of the centre of gravity of the loaded vehicle, and the track width (the distance the tires are set apart laterally). Of less important are such things as the centre of gravity laterally (is the weight of the load more to one side or the other?) and the rigidity of the suspension.

The force trying to push a vehicle over is called lateral acceleration. In a curve, the amount of lateral acceleration is determined mostly by the radius (or sharpness) of the curve, the speed of the vehicle, and the superelevation (tilt) of the road. Rapid steering movements also affect lateral acceleration. This is all pretty obvious.

What isn’t so obvious is something called rearward amplification. Rearward amplification is a lot like “cracking the whip” on ice skates. In a tractor trailer situation, at most speeds the trailer wheels will track outside of the tractor wheels in a curve. In a B-train configuration, the tires of the rear trailer will outtrack even further. This has a dramatic effect on lateral acceleration. In one study of a relatively short (55 foot) B-train during a severe lane change maneuver, the tractor experienced a lateral acceleration of .36 g, with the rear trailer experiencing a lateral acceleration of .54 g. That is why rollovers always start with the rear of the unit going over first, and eventually pulling over the tractor.

A driver in a curve can feel the lateral acceleration affecting his tractor unit. Intuitively he gets a sense as to whether he is going too fast for the curve or not. But, the driver has almost no feel for the lateral acceleration affecting the rear of his unit. The more the articulation points between the tractor and the rear of the unit, and the longer the distance, the poorer the feel.

To complicate matters further the Department of Highways will often post an advisory speed for a curve. These advisory speeds are determined with passenger cars and light trucks in mind, and are extremely conservative speeds for those types of vehicles. In other words, a passenger car or light truck can usually exceed the advisory speed with little ill effect. This cannot be said, however, about tractor-trailer units. With their dramatically lower rollover thresholds, and the higher lateral acceleration that they experience at the rear of their units, the tolerance of an advisory speed is minimal. I have been involved in cases where trucks have rolled over when travelling 75 km/h in curves where the advisory speed was 60 km/h. In these cases, the drivers perceived there to be no danger whatsoever and were completely surprised when their trucks rolled over.

John Drayton is a Kamloops lawyer practicing in the area of motor transport and forestry law.