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Crash Barriers

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Crash Barriers
A30 Goss Moor March 2006 01.jpg
'Armco' crash barriers on the A30 Bodmin Bypass.
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Many of our roads are lined with barriers, also called Roadside Barriers, Traffic Barriers, Guard Rails (in US) or perhaps most commonly Crash Barriers. They serve a variety of purposes, but they are normally installed to either prevent vehicles leaving the road or to segregate two opposing traffic flows on a dual carriageway. Over the years there have been a variety of designs installed, with four main types as below.

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'Armco'

The most familiar type of barrier to British road users is perhaps that commonly called 'Armco'. Any follower of Formula 1 will probably have realised that this is actually the name of one of the companies that produces barriers, but just like 'Hoover', the name has stuck and entered common usage.

The defining design feature of this barrier type is the corrugated profile of the steel. Normally constructed from Galvanized steel, the profile has a vertical double bump with a trough between, and is mounted on posts at regular intervals. The trough can either form a continuous wave curve, or be flat bottomed, with a variety of other subtle design changes depending on the manufacturer. The sections can easily be bolted together to create continuous barrier structures, with overlaps for additional strength.

On main roads, the barriers are mounted on steel posts embedded in concrete, and often with a slot connector or cushion component between rail and post to absorb minor impacts and movements without damage. On minor roads, the barrier can be bolted directly to wooden posts. Whether this is simply an outdated installation process, or acceptable on routes with low average speeds is not altogether clear.

Terminals

Historically, this type of barrier was terminated by being angled down to the ground, and meeting a triangular concrete block (either partially exposed or buried in the ground. However, this led to a few incidents where cars would ride up the 'ramp' making crashes somewhat more serious than they could have been. As a result, in locations where it was deemed a potential problem, the terminations were changed to vertical cushioned or spring loaded pads, which were better able to absorb an impact without lifting the vehicle.

A common solution configuration of terminals on dual carriageways, usually to provide a gap for pedestrian crossing, is for the two barriers to overlap for a period, thus protecting the area where the forward barrier rises, again to prevent cars riding up the forward barrier and turning over.

Beam Barriers

Like 'Armco' type barriers, Beam Barriers consist of a steel barrier connected to a series of posts. The main difference is that instead of the profiled steel, a solid box beam is mounted to the posts. Less flexible in impacts, the beams rely on deformation of the posts, which perhaps brings the vehicle to a halt quicker, rather than deflecting it into the road. The barriers are terminated in a similar manner to those above.

Wire Rope Barriers

A Wire Rope Barrier was used on the Blunsdon Bypass.

A much more recent addition to the roadside, or indeed Central Reservations, is the wire rope barrier, which normally consists of two lengths of metal wire running between poles and criss-crossing each other. They can absorb small impacts with minimal damage to the barrier, although the vehicle may incur substantial damage. They are predominantly installed between carriageways, and less frequently used on verges.

There have been concerns about the impact of a motorcyclist hitting one, so these tend to only be used in England in places where there is no history of motorcycle accidents, such as on modern dual carriageways.

Concrete Barriers

A Concrete Step Barrier being formed on the M74.

The rigid concrete barrier can trace its origins back to the stone Parapets used thousands of years ago on bridges, but technology has advanced substantially in recent years. They are manufactured in sections which lock together when sited and also have solid foundations. The shape of the concrete is designed to absorb impacts with minimal deflection of the vehicle, and also limit the chance of the vehicle climbing the barrier. Furthermore, they tend to suffer only cosmetic damage when struck by a vehicle, meaning there are less ongoing maintenance costs, which is important on heavily trafficd routes such as the M25; however, there have been some high profile failures of the barrier, such as on the M25 at Waltham Abbey in December 2014.

With few exceptions they are installed in the Central Reservations of Motorways and dual carriageways, and in England are almost always inserted prior to All Lane Running or Smart Motorway schemes being implemented. Abroad, these types of barriers are being increasingly used, with large sections of ancient Armco across Belgium having been replaced in the early 2000s, and new schemes in Germany using them.

Temporary Barriers

Temporary Barriers used on the A9 to make a roundabout.

There are also a related group of barriers consisting of sections of interlocking concrete which can be craned into place and form a temporary protection in roadworks or other hazardous locations. Similar barriers sections fabricated from steel have also been used, although these probably have a concrete core to minimise movement. Such barriers are often made noticeable by being alternately painted red and white.


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