John Williamson: A one-size-fits all approach to speed limits doesn't work

Flexible wire-rope centre and side barriers have been installed on the Brynderwyns for the past five years with spectacular safety outcomes. Photo / Michael Cunningham

ON THE ROAD

There was a knock on the door that Sunday morning and there stood a MAMIL- a rather fit-looking but dishevelled Middle-Aged Man In Lycra. He was obviously grumpy, leaning on his bike with his equally grumpy cycling mate. "Why the hell has NZTA just installed cat's-eye reflectors alongside the sideline just up the road", he thundered. "We were head down, going for a time, coming around the corner when we struck these things. Sent me across the road."

I guess the answer was obvious - that particular corner has caused a number of cars to plough into our neighbour's stonewall as well as take out the nearby power pole. So the cat's eyes are there to alert to what's ahead.

This raises the question about how roadside treatments that are designed to make roads safer for drivers, but have a different impact on other road users. In the latest flurry to have a blanket reduction in speed limits across Northland state highways, what seems to have been forgotten is the range of roadside treatments available to make roads safer.

Not so long ago, then Associate Transport Minister Julie Ann Genter affirmed that rumble strips would increasingly be installed on many of our state highways.

These audible, tactile profile markings are designed to warn drivers when they have strayed from the road. They provide drivers with an audible and vibrating message and are installed on the centre line and/or on the sideline of local roads and state highways.

They work, and have been credited with achieving up to 25 per cent of all injury crash savings and 40 per cent of death and fatal injuries where they are installed. They have a benefit-cost ratio of 18.

They are simple to apply to existing roads with little inconvenience to the motoring public and are regarded as highly cost-effective roadside treatments. But they cause problems for cyclists and create noise issues for neighbours living alongside them. Cyclists, though, tend to live with rumble strips once they know where they are. The problem with the previously mentioned MAMIL, is that he didn't know they were there and his preoccupation with the time trial nearly caused his demise.

We have a similar type of discussion with wire rope barriers. AA research indicates 41 per cent of serious crashes involved a vehicle crossing the centre line on an undivided 100km/h road. The most effective way to prevent these, and run-off road crashes, are flexible wire rope centre and side barriers.

These have been installed on the Brynderwyns for the past five years with spectacular safety outcomes. But you can't do a U-turn, emergency vehicles can't pass, cars bouncing along the road can be a problem and there's the motorcyclist cheese-cutter myth.

Both rumble strips and wire rope barriers are proven treatments to make our roads safer with ample research and practical evidence to demonstrate that. But where do speed limit reductions fit into this armoury?

It is well recognised that the default open road speed limit of 100km/h across the whole country, in a one-size-fits-all approach, is quite inappropriate given the range of open roads that we have. Most drivers accept that a safe speed to drive a road depends on the look and feel of the road and the 85th percentile of the driven speed is about where the speed limit should be set. A blanket across-the-board reduction in speed limit of 20km/h will not see driven speed drop by that amount.

There is ample research evidence, dating back to 1992, that dropping the speed limit by itself will not have the attendant road speed reduction. This research tends to indicate that dropping a speed limit by 20 per cent is likely to lead to an operational speed drop of only 5 per cent. This may be all that the regulators are looking for, but it does open another can of worms of enforcement, travel times, vehicle emissions, congestion, driver frustration, and cost increases against possible crash reductions.

Speed limits have got to make sense against the look and feel of the road, and unless the new speed limit is closely aligned with the 85th percentile of the existing traffic speed, there is unlikely to be a balanced safety and economic benefit. There are multiple tools available to make our roads safer- we need to make use of all of them.