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Around a third of heavy vehicle crashes in New Zealand involve a rollover. More than half of these involve truck-trailers. Rollover can happen when you're going too fast for a curve in the road, your loaded vehicle has poor stability or the load shifts or moves within your vehicle during the journey.

Steps you can take to prevent rollover

  • Reduce your speed on curves by at least 10km/h below the recommended speed shown on the yellow warning sign before the curve, particularly if laden. Don't speed up until all parts of your truck and trailer combination have completed the turn.

  • Ensure that your loaded trailer is within the maximum weight and height limits for SRT as shown on the certificate of loading or SRT compliance certificate.

  • Properly secure your load.

Using the static roll threshold (SRT)

Using the SRT will ensure you meet your legal requirements and save on the significant cost to your business of rollover. The threshold evaluates the gross mass of your vehicle against its height or load to give a measure of the likelihood of a vehicle rolling sideways.

What you can do: Make sure your load doesn't exceed the height or weight shown on the certificate of loading or signed SRT compliance certificate (normally carried in the cab). Where the weight on the certificate is lower than the road user charges (RUC) weight or maximum legal road weight, you can only carry the lower weight load.

While SRT must be certified by an HVS certifier with the HVS1 or HVS2 category, the SRT calculator (external link)  is a quick and easy tool that may be used as an indicator to give guidance that a load is legal and safe for the vehicle classes listed below - although it may not be used for compliance or enforcement purposes.

Find out more on the specific requirements for your vehicle including conditions, certification requirements and implementation dates.

Read the Heavy vehicle stability guide to learn more about how you can reduce the risks of rollover and crashes caused by loss of control.

Heavy vehicle stability

Take these steps to reduce the risk of rollover.

1. Reduce speed around curves

Even a small increase in your cornering speed results in a much larger increase in the outward sideways force which can tip your vehicle over.

Diagram of outwards sideways force What you can do:

  • When unladen, reduce your speed to that shown on the curve warning sign.

  • When laden, reduce your speed on curves to at least 10 km/h below the posted speed shown.

When turning at a roundabout or intersection, stay at low speed until both your truck and its trailer(s) have fully completed the turn and are straight behind each other on the exit road.

Curve warning signs showing 65km/h

Curve warning signs

2. Don't overload your trailer

Check page 2 of your trailer's certificate of loading for a line that looks like this:

Static roll threshold
0.35 g, 4.25 m/15 t, 18 t/3.70 m

This shows the height/weight load limits under the SRT. In this example:

  • if the trailer is loaded to its maximum height of 4.25 metres from the ground, the gross weight of its axles must not exceed 15 tonnes

  • if the trailer is loaded to its maximum legal gross axle weight of 18 tonnes, then its maximum load height must not exceed 3.70 metres from the ground.

3. Make sure your load is secure

Take extra care if your vehicle is carrying loads that are prone to moving during the journey, such as:

  • livestock

  • hung meat carcasses

  • bulk liquids in tanks.

What you can do: Learn the right techniques to secure these loads and keep partially loaded compartments to a minimum.

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