Best practice investigating lorry rollover

What is vehicle rollover?

For any given vehicle travelling on a curve, there is a sideways acceleration threshold above which it is prone to roll over. The sideways acceleration it experiences is determined by its speed and the curvature and the camber or crossfall of the curve. Critical to a rollover happening is the height of the centre of gravity of the vehicle, placing lorries at greater risk compared to cars and other low vehicles. Articulated lorries are at an even greater risk due to their semi-trailer, which is the most unstable part of the vehicle.

Vehicle rollover occurs during cornering because the sideways force acting through a vehicle’s centre of gravity causes it to lean. A useful metric for understanding a vehicle’s susceptibility to rolling over is the Static Stability Factor (SSF), which is the ratio of half the track width of its wheels to the height of its centre of gravity: the lower this ratio is, the greater the risk of rollover. However calculating the SSF does not immediately yield the rollover threshold since there are other factors which affect, and generally lower, it.

What are the causes?

A vehicle’s rollover threshold is also affected by a number of factors including load placement, suspension and even tyre pressure . If the load of a vehicle is distributed unevenly, or should it slip sideways, this will reduce the rollover threshold in that direction. It might be that a load which was originally evenly distributed slips sideways at the start of a turn because it was inadequately secured, leading to a rollover which would not have happened had the load been held in place.

In most commercial truck operations, the load on the vehicle is fixed and nominally centred. In certain cases, however, the load may be able to move in the vehicle – such as liquid cargo. A compartment that is filled to anything less than its full capacity allows the liquid to move from side to side, producing a “slosh” load condition. This can reduce the vehicle’s performance in cornering, thus further reducing the rollover threshold.

There is often an allegation in rollover investigations that the load was not secured properly. If true, this would indeed lower the rollover threshold and make a stable vehicle unstable. However, it can be difficult to know for certain because when a vehicle rolls and lands on its side, the load can move. The question then is did this happen before or after the roll?

How can we prevent it?

Vehicle rollover costs a great deal of money – recovering vehicles, repairing roadways and replacing damaged loads are all expensive activities. In serious cases, there is also the moral cost of serious injury or death. Prevention of vehicle rollover primarily focuses on altering driver behaviour – such as promoting slower speed, regular load and tyre checks and avoiding abrupt steering movements. Improvements to basic vehicle design, to lower the centre of gravity, would also help in reducing rollover.

What role can TRL play?

There are many complicated factors involved in lorry rollover such as vehicle size, speed and load type. For insurers and legal representatives, it is critical that conclusions around the circumstances of vehicle rollover are accurate as the possibility of a wrong determination anywhere in the investigation process and subsequent legal proceedings can cause significant repercussions.

As a result, it is vital that we educate not only the drivers, but also those involved in investigation. The right expertise must be deployed across the entire chain, especially so in complex areas such as the calculation and application of threshold formulas.

Here at the TRL Investigations Team, we have the capability of analysing an incident and ascertaining whether it was caused by faulty loading, load slippage or driver error. We can examine tachograph recordings, CCTV and GPS data, plus the curvature of the vehicle’s path and the camber of the road to reach an accurate conclusion.

If you are interested in finding out more about this service, please get in touch with us at: investigations@trl.co.uk