Think of safety barriers and the chances are you think of armco, a name that harks back to the American Rolling Mill Co of the nineteenth century. We’ve grown up with it, and its use is almost universal. But just as the high profile world of Formula One has seen changes in the containment methods for errant vehicles, so too has the everyday world of our main trunk roads and motorways. Readers will have noticed the presence of ‘wire rope’ fences between carriageways plus the occasional section of solid concrete barrier, and wondered what was wrong with the old armco.
There are three basic types of central barrier: wire rope, steel, and concrete. Steel beams and concrete barriers are available in a wide variety of styles. The steel for instance can be TCB (tensioned corrugated beam) which we know as armco, or OBB (Open Box Beam), while the concrete comes in numerous shapes and sizes. The big question of course is “which of them is best?”
To say that it’s not too straightforward a decision would seriously understate the case. Here’s an extract from the Highways Agency wording: “the choice of safety barrier is based on the individual characteristics of a route, for example the type of vehicles that use it and any associated hazards.” It goes on to say that what may be appropriate in one area, a concrete barrier perhaps, may not suit another, such as a central reserve on a route mainly used by small vehicles. There are of course standards which all barriers must pass, and the Highways Agency lists all those that do. If you like lists you’ll like this one – the safety barriers listed in the ‘Highways Agency Accepted EN1317 Compliant Road Restraint Systems’ take up no less than 36 pages.
Wire rope barriers have been around since the sixties. They were developed for our northern climes to combat the problem of drifting snow that built up around the more solid barriers. It’s certainly the least expensive of the barrier types, with concrete being the dearest. But ongoing studies of whole life costs – as opposed to mere installation – do put concrete in a more favourable light. You can get the full story with real-world costs and accident statistics on the Department for Transport website: www.dft.gov.uk/ha/standards/tech_info. Look for Median Safety Barriers and their Whole Life Cost-Benefit Analysis. There you’ll find the Transport Research Laboratory’s in-depth look at the case study area of the ‘M25 sphere’ (including a few miles of the motorways radiating from it), where the Highways Agency has its greatest concentration of concrete barriers. At the time of the survey these constituted 12.7 per cent of the total. Amongst the nuggets we can read is the sobering news that with no damaging impacts, one kilometre of double-sided armco has a 50-year whole-life cost of over £430,000 (2007 prices), which is almost ten per cent more expensive than a ‘vertical concrete barrier’. This is simply because over those fifty years the armco will need two installations and removals compared to the concrete’s one.
So cost benefits of armco are very likely only short term ones, but the more important control of barrier type is the safety aspect. And how safe a particular barrier is will depend on what hits it. So analysis of the vehicle types using a stretch of road is a vital consideration when making the decision. The same survey referred found that of accidents in the M25 sphere between 1990 and 2002:
• No deaths have resulted from an impact with a concrete barrier;
• The number of serious casualties per kilometre is comparable between steel safety fencing and concrete safety barriers;
• Concrete barriers result in a lower rate of slight casualties and total accidents per kilometre than metal safety fences.
So the latest research indicates that concrete barriers are possibly safer than the other types, and in the long term less expensive. Not surprisingly therefore, on motorways or roads constructed to motorway standard, where the two-way Annual Average Daily Traffic is 25,000 vehicles per day or more, it is now Highways Agency policy that all central reserve barriers are made of concrete – unless weather conditions dictate that a wire rope barrier is more appropriate. More importantly perhaps, the Highways Agency tells us that existing steel and rope barriers “would be upgraded to concrete on motorway-standard roads (with AADT>25,000) when routine replacement is required or as part of an upgrade programme.”
The case for non-motorway dual carriageways is less clear cut. To quote them again: “Concrete barriers are not mandatory on trunk roads, but subject to risk assessment, and there are still cases where they may be used to good effect.” They’re definitely talking traffic volumes and speeds here. And cynics might suppose another factor would be the amount of cash in the kitty.