Wire rope barriers shown to reduce serious crash incidence by up to 87%

By intouch * posted 16-03-2016 15:28


Loss-of-control crashes – where the driver veers off the edge of the road or into oncoming traffic – are a major source of serious road trauma. Although there are a variety of road safety barriers on the market, researchers have pointed to wire rope barriers as the safest option for the majority of road users.

Researchers from Monash University Accident Research Centre (MUARC) looked at 100 km of wire rope safety barriers (WRSB) installed on Victorian roads. They found the barriers significantly reduce the incidence of all crashes, and run-off-road and head-on crashes, with estimated reductions on individual routes ranging between 75% and 87%.

The report found these percentages were consistent with overseas findings.

Report author Nimmi Candappa explains that, of the crash barrier options available, pre-cast concrete jersey barriers and posed the greatest potential for injury to motorist on impact, followed by steel W-beam or Armco barriers.

“Our research indicates that barriers like the concrete barriers are not overly good for road safety simply because they're so rigid,” she says. “Whatever impact force occurs at a crash, most if not all of the force goes straight back into the vehicle and to therefore the occupants.”

“Armco barriers are in-between; they're semi-rigid. There's still a high risk of occupants sustaining injury when colliding with this guard rail. There's also some situations where the guard rail can actually pierce the vehicle.”

The impact-absorbing ability of fractable wire rope barriers means they are generally regarded as the safest option.

“Essentially they're highly-tensioned steel cables. There are either four parallel or sometimes three parallel or two parallel and two intertwined cables, depending on the design being used. They essentially sit on steel posts and the ends are tensioned so that they form a barrier in terms of the rigidity. As soon as a collision occurs, the post falls apart and then the cables absorb a lot of the impact force.

“From a physics or the crash mechanics point of view, this is the safest form simply because, once there is a collision, there's no rigidity in the barrier and the impact force is absorbed by the barrier rather than by the vehicles." 

However, this advantage in safety does not extend to motorcyclists. Candappa explains their smaller mass is often not enough to collapse the poles, meaning the barrier remains rigid on impact.

“The issue from the motorcyclist's point of view though is they feel as though, because the posts can often be sharp on the edges, they might actually snag them in some way and impart greater injury than the smooth side of the concrete barrier.”

While Candappa concedes this may be the case when a motorcyclist slides into a barrier at low speed, her research has shown there are no truly safe options for motorcyclists that will also be effective for larger vehicles.

“A motorcyclist will sustain injury whether they crash into a concrete barrier, a guard rail, a wire rope barrier, or a tree that we're trying to protect them from,” Candappa says. “It will still impart a certain amount of force back onto the motorcyclist.”

In countries such as Australia and New Zealand, where larger vehicles outnumber motorcyclists, Candappa says the road authorities have a responsibility to cater to the majority of road users.

“I think it would be questionable if we disregard a barrier or some form of infrastructure that is considered really, really quite good for safety for the majority of road users in the hope of catering for a very small percentage of road users,” she says.

While there are a number of products such as pole covers and slide bars that can give added protection to motorcyclists without negating the benefit for other road users, they come with a financial sting in the tail for road authorities.

“It becomes harder to put these barriers en mass on the roads,” Candappa says. “We need to fine tune what the actual design is and what the cost will be for long lengths of these barriers.”

“I was speaking to someone in Malaysia about whether they are kind of increasing their length of wire rope barriers and he said their their percentage of motorcyclists is about 50%. Naturally they had to be a lot more careful about what suits the motorcyclists compared to the rest of the passenger vehicles.

“It’s an example of when catering for the larger proportion of road users is important.”

Wire rope safety barriers explained  

Ingal Civil Products, Product Manager Luke Gallagher says WRSB are suitable for a very broad range of road environments but are particularly suited to relatively straight roads or roads with large radius bends. "The limiting factor in their use is very tight curves which can be in the horizontal or vertical planes," he says. 

Gallagher says the systems work in a similar way other semi rigid barriers, such as guardrails. "The interaction begins with vehicle engaging the barrier and some deflection of the barrier will occur, the resistance to deflection is what contains and redirects the barrier back towards the road and away from the hazard," he explains.

"In WRSB systems, the cables do the most of the work in the redirection of the vehicle and these cables have a really high capacity to absorb significant impact energy. In a concrete barrier there is also high capacity but no deflection and this rigidity creates a very violent impact where all the impact loads are absorbed by the impacting vehicle. WRSB systems share this load and offer a much softer and lower risk impact on the vehicle occupants.

Photos: Courtesy of Ingal Civil Products