Special vehicle advisory speeds

There may be situations where specific vehicle types need to be taken into consideration when developing advisory speeds.  One of those situations is the significant risk of trucks towing heavy trailers rolling over on curves. The provision of accurate advisory speed signs is necessary for the safe operation of all vehicles but truck/trailer safety and stability is highly sensitive to the speeds at which curves are traversed.

Stability performance standards

The Land Transport Rule: Vehicle Dimensions and Mass 2002 introduced a stability performance standard for the heaviest classes of trucks and trailers, which must have a Static Rollover Threshold (SRT) of at least 0.35g whenever used on a road.

Land Transport Rule: Vehicle Dimensions and Mass 2002

New Zealand research confirms an SRT of at least 0.35g is required to negotiate curves at the posted advisory speed. Because advisory speeds are signed at 10km/h intervals the speed on the sign should be within ±5km/h of the measured advisory speed. When the measured advisory speed is at the lower end of this 10km/h interval, particularly at low advisory speed values (ie 15, 25, and 35km/h), a large vehicle may generate a lateral acceleration higher than the minimum SRT requirement and become unstable at or near the advisory speed value.

On curves, drivers of heavy vehicles need to reduce their speed on curves to below the values shown on speed advisory supplementary signs, particularly when towing heavy trailers.

Setting appropriate special advisory speeds

In relation to setting appropriate truck advisory speeds an RCA should review advisory speeds to ensure that they are determined in accordance with current standards and are within 5km/h of the measured advisory speed.

Truck driver responsibility

Waka Kotahi NZ Transport Agency preventing rollover guidance advises truck drivers to:
Preventing rollover

• reduce speed, especially on curves; and
• go at least 10km/h below any advisory speed sign posting on curves.

It should be noted that the concerns about truck/trailer stability and advice to heavy vehicle drivers to travel at least 10km/h below advisory speeds, reinforces the need for accurate values on Curve sign supplementary – advisory speed signs.

Signs

Where a need for a special advisory speed sign has been determined, refer to Table 3‑9. 

Table 3‑9: Special vehicle advisory signage

Table 3‑9 notes:

1. The sign should be located where approaching drivers have an uninterrupted view of it over a distance of at least 120m in rural areas or at least 60m in urban areas.
2. The sign should be installed in advance of the curve by at least the distance shown in the table below.