New Zealand has a high incidence of rollover and loss-of-control crashes for distance travelled compared with countries such as the USA and Canada. This reflects our difficult driving conditions. We have more bridges, corners and hills per 100 kilometres of road and fewer divided highways than most other developed countries. Drivers of large vehicles have an obligation to drive considerately and professionally at all times. Rollovers in single-vehicle crashes involving trucks often reflect drivers’ lack of appreciation of the general dynamics and stability issues of large vehicles.
There are many factors that influence a vehicle’s tendency to rollover, but the following are the most crucial:
If all these factors are contained within acceptable levels, the vehicle will remain stable. If they are not, then the risk of loss of control and rollover will increase.
Typically rollovers happen during cornering (at either low or high speed) and during sudden evasive steering manoeuvres.
Vehicle dynamics: this term refers to the motion of a motor vehicle and the various forces that act upon the vehicle when it is in motion.
Side wind: vehicles with high flat sides, such as furniture trucks, are extremely susceptible to the effects of crosswind. Be aware of places where there might be strong crosswinds. Listen to weather warnings and act accordingly.
Remember your vehicle will generate its own wind as it moves along the road. The faster you go the greater the wind force generated. This could seriously affect cyclists, motorcyclists and other vehicles and cause them to lose control. Be considerate, slow down and give them room.
Speed: the effect of speed on the cornering ability, braking distance and impact forces acting on a vehicle increases as the speed increases. When speed doubles, cornering forces increase by four times.
Centre of gravity: the stability of a truck is largely dependent on the height of its centre of gravity above the ground. The higher the centre of gravity the more unstable the truck will be. If a load is not centred across its width the stability will be reduced when cornering. If the load is not balanced correctly along its length, wheel lock-up during braking becomes a distinct possibility. When wheel lock-up occurs the vehicle loses steering control.
Kinetic energy: kinetic energy is the energy present in any moving object. The heavier and/or faster the object, the more energy it will contain. The effects of kinetic energy increase at the square of the speed and have a major influence on all motor vehicles in three particular situations, as explained below.
Braking: the faster the vehicle goes, the further it takes to stop. If a truck’s speed is doubled it will take at least four times the distance to stop and this will be longer if the road is wet.
| Truck speed | Weight | Road surface | Stopping distance |
| 35km/h | 39,000kg | Dry | 9 metres |
| 70km/h | 39,000kg | Dry | 37 metres |
| 70km/h | 39,000kg | Wet | 72 metres |
Cornering: if a truck enters a corner at 60km/h, there will be four times more overturning (side) force on the vehicle than if it entered the corner at 30km/h.
Impact: the damage to a vehicle (and its driver) at the point of impact in a crash situation will also increase in proportion to the square of the speed:
