31 October 2018

Concrete vs Wire Rope Safety Barrier – what’s more effective?

As our road toll continues to climb road safety experts, both here and in Australia, are constantly looking at the safety systems at their disposal to ensure motorists are kept safe at all times - even when an accident occurs.

Flexible wire rope road safety barrier systems (also known as WRSB) are being increasingly specified in median and side of road installations across the country by the NZ Transport Agency and Councils as evidence continues to prove that WRSB systems are far more forgiving on the human body, in both car and motorcyclist incidents, than any other type of barrier.

WRSB systems are designed to prevent traffic leaving the road or crossing the centre line and colliding with hazards such as other vehicles, trees and poles by 'catching' and then redirecting vehicles. When a vehicle impacts the wire rope safety barrier the cables flex, slowing the vehicle and pushing it back into its lane with the system absorbing the impact.

Flexible road safety barriers are narrow and work best on long, straight sections and gentle curves. More importantly though, the NZ Transport Agency says that, when fitted along the centre or the side of the road, they reduce the number of people killed by 70-80%.

‘Cheese cutter’ or not

“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,” says Voytek Wieczorek, Engineering and Technical Support Manager for CSP. “However studies have shown this assumption is incorrect. Motorcyclists are more likely to survive an impact with a flexible road safety barrier than impacts with trees, poles, concrete barrier or oncoming vehicles which the barrier will prevent them striking in a crash.''

Wire Rope Barrier comes out on top

In research undertaken, the safety performance of road barriers in reducing the risk of injury was assessed, with hazardous events such as rolling over, striking three types of barriers (guardrails, concrete barrier walls, and cable barriers) with different barrier offsets to the edge of the travelled way, and striking various roadside objects being studied.

A total of 2124 single-vehicle crashes (3257 occupants) that occurred between 2008 and 2012 were analysed in a study in the USA.

The study found that the odds of injury are:

  • 43% lower when striking guardrail over a median concrete barrier (offset 4.5 – 5.5m)
  • 65% lower when striking a median concrete barrier (offset 2 – 4m).

The odds of injury when striking a near-side median cable barrier are:

  • 57% lower than the odds for a guardrail face.
  • 37% lower for a far side median cable barrier.

In other words - guardrail should be preferred over a concrete wall and wire rope cable barrier should be preferred over guardrail - where the road and traffic conditions allow

For more detail on the study click here.

“Although this study has its merits, road conditions in New Zealand and Australia are quite different to the USA,” says Julian Chisnall, NZ Transport Agency. “For instance we have a lot more curves in our roads - particularly in rural areas. You will see a lot of concrete barrier on motorways where the angle of impact is likely to be less in an accident. We tend to use concrete on motorways. However in the case of the Waikato Expressway, where we had the room to create a large median strip, we have used a lot of WRSB. It’s certainly much more forgiving on the vehicle occupants in a crash.”

For a report from Transport and Road Safety (TARS) on Motorcycle Crashes into Roads Safety Barriers dated December 2014, click here.

Over the years Brynderwyn Hill on State Highway 1 north of Auckland has been a notorious hot spot for motor vehicle crashes. In late 2014 the NZ Transport Agency, as part of the Safe System project, instigated the widening of the road, the removal of tight corners and the installation of 14kms of wire rope barrier using CSP’s Brifen WRSB system – to separate north and south bound traffic from the top to the bottom of the hill.

CSP’s Brifen Wire Rope system was chosen over others due to its interwoven cable system which is best suited to curved installations. When the barrier is impacted the interwoven cable system ensures the tension is kept on the cables at all times, even on tight radii.

In the year to 31 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 NZ Transport Agency says that a serious crash has been avoided, and someone has been able to drive or walk away from the incident. Each time the centre line barrier is damaged a potential head on collision has been avoided.

Armorwire – the easiest and quickest WRSB system to install

The Rangiriri section of the Waikato Expressway opened in late 2017 and features more than 20 kms of road barrier - 14kms of which is CSP’s Armorwire Wire Rope Safety Barrier and 7kms being CSP’s Nu-Guard® 31 Guardrail System.

“We have been involved in various sections of the Expressway for the past 10 years or so,” says Daniel Hoskins the contractor responsible for road barrier installation. “I find Armorwire the easiest and quickest to install of all the Wire Rope Safety barriers out there. Armorwire is a really good system.”