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Collision Division

Crash barrier technology has been a hotbed of debate since 2005 when the Highways Agency ruled in favour of concrete. Much has changed since then, discovers Kevin Walsh.

The last thing the average English motorist wants to think about is his or her survival prospects if they crashed into a roadside barrier at 70mph.

This of course is left to the Highways Agency which decided in its 2005 Interim Advice Note (IAN) that all new crash barrier installations in the central reservation of motorways should be made from concrete.

The motivation for this was to reduce maintenance costs and eliminate the most devastating kind of motorway accident – the crossover – where an out of control vehicle smashes through the barrier and into the opposite carriageway.

Accidents of this kind are three times as likely to result in death as any other on the motorway.

So far, concrete barrier systems have had an enviable track record since their introduction in the UK 10 years ago. There have been zero fatalities in accidents involving a collision with these barriers.

They are designed to H2 standard, so they are tested to ensure they can successfully contain a 13t vehicle travelling at 50mph or a 900kg car travelling at 60mph. Most steel crash barriers are rated N2, and can only stop a 1.5t vehicle travelling at 70mph or a 900kg car travelling at 60mph.

Also when compared to the more common steel crash barriers which need large stretches replacing after a collision, concrete barriers require little or no maintenance.

Concrete step barriers, sometimes known as Dutch Steps due to their initial use in the Netherlands, have a stepped profile which widens towards the bottom. They are designed so that most of the impact is taken by the lower side of the vehicle, thereby offering some protection to the passenger.

However the main focus is on containment, and the rigidity of the concrete barriers is far more effective than steel at redirecting a car back in the direction of the flow of traffic.

Despite the track record of concrete step barriers, there has long been disagreement within the industry about whether they offer enough protection to motorists.

Steel barrier manufacturer Corus' senior development engineer Rob Philips comments: "It's been an ongoing debate since before I took up this role."

While concrete barriers are undoubtedly better protection against crossovers, their lack of ductility increases the likelihood of injuries to the driver of the errant vehicle.

This is in contrast to steel barriers, which were designed to provide a certain amount of "give" so that the car does not come to quite an abrupt stop after colliding with the barrier. But this comes at the expense of a lower containment capability when larger vehicles hit the barriers at high speed.

Philips is a proponent of the traditional steel safety barriers and believes greater use of them can still be made in central reservations.

"We know that a concrete structure is very hard but there's no energy taken out of the travelling car in a crash, so there's little protection for the driver."

Philips' 22 years working with metal has left him confident that steel barriers currently available can be used to prevent crossovers, even for larger road vehicles, as the strength of the barriers is dependent on how much steel is used in their construction.

This would allow for a level of compromise between protection of the driver of an errant vehicle and those on the opposite carriageway.

Corus' Vetex barrier system is available at different working widths (a measure of the amount of deflection of the barrier allows in a crash, plus barrier width) and can be designed to the H2 standard.

Although these H2 steel barriers are not double-sided and would require twice as much material for a motorway central reservation, Philips surmises: "I wouldn't want to crash into anything but if I had to choose, I think I'd rather it be into a steel one."

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