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Fact Sheet

6th of June 2011

Road Damage

Have you noticed ruts in the roads at the traffic lights on the Western Hills bypass and through Whangarei?
 The ruts are very likely to be caused by large, fully loaded trucks.

Road planners use the “Fourth-power” rule of road wear. According to the rule, if you double the load on an axle, the road damage is increased by a factor of 16 (2x2x2x2=16) (Wikipedia, 2010). This implies that a standard 44 tonne 8 axle truck (current maximum size in New Zealand) does 20,000 times more damage than a car. Therefore the larger 53 tonne trucks that the New Zealand government is proposing to allow on the roads will do 42,000 times more damage than a car.

A computerised model that looked at the potential closure of 800 miles of railroad branch lines servicing the grain silos in Western Kansas indicated that road damage before abandonment was $2,090,107 and after abandonment was $3,094,497. This is a 48% increase in damage (Bai, 2009).

It is likely that this research would apply to Northlands situation.
If the Northland railway was to close, then the increased truck volume would equal the increased damage sustained by the roads. There is likely to be a 10% increase in numbers of truck travelling from Whangarei to Auckland, which would result in 10% more damage being done to this road every day.


Bai, Y., Schrock, S., & Mulinazzi, T. (2009). Estimating Highway Pavement Damage Costs, Attributed to Truck Traffic. Retrieved August 31, 2010, from http://www.iri.ku.edu/publications/HighwayDamageCosts.pdf

Wikipedia, (2010). Gross axle weight rating. Retrieved August 31, 2010,

Written by Kelvin Taylor. Save Our Rail Northland. Contact Email kelvint@ihug.co.nz

Road Congestion

Road congestion is a fact of life or is it? There are more and more vehicles on the road every year.

Large trucks have a much bigger effect on congestion than cars because they are longer, less manoeuvrable, and underpowered compared with typical car. They accelerate more slowly; need larger gaps, more lane width, more time to make turns and may slow down on long grades (TRB 1990).

At intersections, a large truck can represent 3 to 4 passenger cars equivalents [i.e., a large truck equals 3 to 4 “passenger car equivalents (PCEs)].”  The effects of trucks at intersections is very noticeable with logging trucks travelling south through Whangarei at the Western Hills, Maunu Road intersection.

Increasing the percentage of trucks in the general mix of traffic therefore can cause a marked reduction in capacity (TRB 1990). For example, if 10 percent of the vehicles are heavy trucks on a route with signalled intersections, capacity will drop 20 to 25 percent (TRB 1990).

On a level, multi-lane highway (like Auckland motorways), a large truck is equivalent to 1.7 PCEs. If there are steep grades or sustained grades, the trucks will slow down and represent 8 PCEs on freeways or even more on 2-lane highways where passing opportunities are limited.

This indicates that the issues with the State Highway 1 from Whangarei to Auckland are the hilly sections of road and the sections with limited passing opportunities. Since this road is hillier and less developed, compared to other sections of State highway 1, a relatively small increase in truck numbers will have a greater effect on the road congestion.

Currently, every day, there are between 1000 and 1200 truck (9000 vehicles) that travels on State Highway 1 between Auckland and Whangarei. There are another 86 trucks of freight carried by rail (NRC, 2007).

The volume of freight has been predicted to increase by 100% by the year 2030. This will have serious implications for the Northland economy due to the slowing down of transport because of increased road congestion. The upgrade of the Northland rail line will help to alleviate the congestion problems.


Transportation Research Board. (1990). Providing Access for Large Trucks
Special Report 223, Washington: National Academy Press

NRC. (2007a). Heavy Traffic Volumes Report. Retrieved May 20, 2010, from http://www.nrc.govt.nz/upload/1575/Heavy%20Traffic%20Volumes%20Report%20Feb%202007%20chp.%201%20-%205.pdf

Written by Kelvin Taylor. Save Our Rail Northland. Contact Email kelvint@ihug.co.nz


Subpages (1): Kelvin Taylors Report