Many NZ roads not up to speed limits

A heavy weekend road toll has sparked debate about how to improve road safety in New Zealand – is lowering the speed limit the answer?

In Sweden, a world-leading nation in terms of road safety, a ‘‘Vision Zero’’ guides policies.

This is underpinned by a philosophy that death and severely disabling injury are not an acceptable price to pay for mobility.

Such a vision encompasses a safe vehicle fleet accompanied by forgiving road design and appropriate speed limits.

The speed limits are set most obviously as a limit for speed, but less obviously as a limit to the amount of trauma the human body can sustain when subject to crashes at that speed.

We can learn from Sweden’s logical approach to speed limits.

If we set speed limits according to the amount of trauma the human body can sustain we need to consider the types of crashes that might occur on the road. In areas where there are many children, who are prone to behaving unpredictably, you want to set your speed limits so that children are not killed, even if they dart in front of an unsuspecting driver.

There are other crash types that need to be considered.

Generally, occupants of a vehicle hit from the side will suffer much severer injuries than those in a vehicle hit from other angles. There is much less space between the occupants and the collision point. This means that on roads where vehicles commonly interact at right angles (where there are intersections, driveways and access- ways) there should generally be lower speed limits than roads that have few such interactions.

Vehicles that leave the road and hit fixed objects, such as trees and power poles are also liable to produce very severe injuries to drivers and passengers. This means roads that are narrow, with many curves, or with many adjacent objects should be subject to lower limits.

Vehicles that collide with other vehicles head- on also produce nasty consequences not only to a vehicle that might cross the centre-line, but also to the other vehicle into which they collide. This implies that roads without a median barrier should also be subject to lower speed limits than those with such barriers.

There are many roads in New Zealand where 100kmh is the speed limit, but the roads rarely accommodate such speeds safely. These roads have many of the features mentioned above that can produce severe and fatal crashes, typically narrow, allowing little room for misjudgment, with multiple intersecting roads and access-ways. They may have little traffic, which can create a false sense of security where drivers think they are the only vehicle on the road.

So do these safety considerations mean that we need to drive on our rural road network at snail’s pace?

Vision Zero is something that can be aimed for, orienting road safety and roading decisions; but a zero road toll would necessitate severely constrained speeds on all New Zealand roads at the moment. Nevertheless, the default rural road speed limit of 100kmh is not rational. It does not reflect the considerably higher risks imposed by an important proportion of the road network.

Dr Michael Keall is research associate professor in the Department of Public Health at the University of Otago.