Wire safety barriers on the Brynderwyn Hills south of Whangārei are being credited for preventing almost 40 potentially serious crashes and possible deaths since they were installed three years ago.

NZ Transport Agency started installing the flexible road safety barriers on State Highway 1, on the northern side of the Brynderwyns, in August 2015 as part of the $18 million Brynderwyns Improvements Project.

NZTA director of safety and environment Harry Wilson said the barriers have worked with dozens of potentially serious crashes avoided since they were installed.

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''The barriers have been a lifesaver ever since, with only two serious injury crashes - in 2016 and 2018 - and no fatalities,'' Wilson said.

''In the year to the end of July 2018, the barriers were hit 37 times with enough force to require repairs to the wire rope and replacement of fence posts. Each time the barriers are damaged we know that a serious crash has been avoided, and someone has either driven or walked away from the incident.

''Each time the centre line barrier is damaged we know a potential head on collision has been avoided. Head-on and run-off-the road crashes account for 75 per cent of crashes on rural state highways.''

He said the Brynderwyns project improved safety by widening the road and shoulders, removing tight corners and installing 14km of safety barriers along the edge of the road and the centre line to separate north and south bound traffic.

Wilson said from 2006-2010 there were five fatalities and four serious injuries on this section of road. In May 2010 the speed limit was lowered from 100km/h to 80km/h, which reduced the number of crashes. There was one serious injury crash in 2013 and only two serious injury crashes since the barriers were installed.

''Flexible safety barriers are designed to stop traffic leaving the road or crossing the centre line and colliding with solid objects such as other vehicles, trees, poles and ditches. This helps creates a more forgiving road that can help stop simple mistakes becoming fatal crashes,'' he said.

Fatal and serious crashes were a regular occurrence on the northern side of the Brynderwyns, above, but they've been reduced since wire safety barriers were installed in 2015. Photo / file
Fatal and serious crashes were a regular occurrence on the northern side of the Brynderwyns, above, but they've been reduced since wire safety barriers were installed in 2015. Photo / file

Flexible road safety barriers 'catch' vehicles which are going off the road. When a vehicle hits the wire rope safety barrier the cables flex, slowing the vehicle and pushing it back into its lane. This flexibility means that the barrier absorbs impact energy, reducing the force on the people in the vehicles, resulting in less severe injuries than other safety barrier systems and from collisions with roadside hazards or other vehicles.

''This is the best possible outcome for road users and proof that these barriers are highly effective in preventing deaths and injuries. We're really pleased to see the benefits of these safety improvements paying dividends,'' Wilson said.

NZTA said it is not the case that the wire barriers are more dangerous for motorcyclists if they crash

''Motorcyclists have been opposed to flexible road safety barriers because they think the steel ropes will act like a 'cheese cutter' when hit by a rider. However, studies have shown this assumption is not correct. Motorcyclists are more likely to survive an impact with a flexible road safety barrier than an impact with trees, poles or oncoming vehicles which the barrier will prevent them striking in a crash.''

Northland's Regional Transport Committee chairman John Bain said there was no doubt the flexible barriers had saved lives on the northern side of the Brynderwyns, but the big problem now was on the south side, which had no such barriers nor the room to install them.

''The safest flexible barriers I've seen anywhere in New Zealand are on the Hamilton-Cambridge expressway and it's a four lane highway where they have increased the speed limit to 110km/h because it is deemed to be so safe,'' he said.

''I'm sure there are times the barriers have stopped people crossing the centre lines and averted disaster, but the reality now is that the main problem is the south side of the Brynderwyns, where a recent truck accident blocked the road for eight or nine hours.''

Bain said the transport committee saw four laning SH1 from Whangārei to Kaiwaka as the best possible road safety measure for the region.

He said the highway will bypass the Brynderwyns at some stage, but until that time there will still be people crossing the centre line on the road south of Whangārei and going into on-coming traffic and four laning will make the highway much safer.