Crash Barriers
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Many of our roads are lined with barriers, properly called Vehicle Restraint Systems, but also known as Safety Barriers, 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. Parapets are a special form of Vehicle Restraint System
Design Principles
In response to the Selby rail crash, where a vehicle left the M62 motorway ahead of a bridge over the East Coast Main Line near Selby, ending up on the track where it was struck by a train, a risk assessment based process was introduced by the then Highways Agency to determine the need for Vehicle Restraint Systems on roads. The system is detailed in DMRB but in simple terms it considers factors such as the alignment of the road, speed and nature of traffic using the road, the presence of hazards next to the road, including unprotected drops, bridge abutments or piers, lamp columns, and anything else that may cause harm if a vehicle leaves the main carriageway.
The process will determine the level of containment required, categorised as Normal or High Containment, as well as the working width requirement for the barrier system. The working width is effectively the distance that a barrier is free to move without hitting another object in the event of a collision and can be classed from <0.6m for class W1 to between 2.5 and 3.5m for class W8.
Along the edge of a high speed road, different features may necessitate different levels of containment and working widths - for example in an area with a wide verge and obstructions set back 3.5m from the edge of carriageway requires a lower level of containment than a bridge abutment set 1.2m from the edge of the carriageway. Therefore, barriers will have transition lengths where the strength of barrier changes from one containment level to another. This could be a change of barrier type, or it could be that the barrier is stiffened by increasing the number of posts.
The desing process also considers terminal and entry/exit taper design. A vehicle hitting a barrier at the start of the system will not be contained in the same way as it would further along the barrier. Therefore, there needs to be an entry taper over a length of the order of 30m ahead of the first thing to be protected. There also needs to be a terminal detail so that the occupants of a vehicle hitting the end of the barrier head on are not put at risk by the collision.
The complete barrier system therefore comprises a terminal, entry taper, main barrier system with transitions between different levels of containment, followed by an exit taper and a final terminal.
Over the years there have been a variety of designs installed, with five main types as below.
Untensioned Corrugated Beam (UCB) System
These barriers, made by companies such as Armco, are generally used in very low speed environments off road, for example in car parks. They are made from a series of posts connected with corrugated beam sections and look very similar to, but are very different from the barriers found in the central reservation of many high speed roads. Historically, untensioned beams have been used on adopted highway, sometimes mounted on wooden posts. These systems are not tested and are no longer used on public highway.
Tensioned Corrugated Beam (TCB) System
The most familiar type of barrier to British road users is correctly called Tension Corrugated Beam, but is called 'Armco' by many people, though Armco do not make a TCB product. 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 steel 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 are bolted together to create continuous barrier structures, and are then tensioned by tightening a series of bolts at intervals along the barrier. The tension in the barrier makes them far stronger than the similar looking untensioned systems. These systems are tested and rated to various levels of containment, depending on post spacings. Posts spaced closer together make the barrier stiffer with a higher level of containment and a smaller working width.
The TCB barrier can be installed as either a single-sided or double-sided system depending on whether it is used on the verge or in the central reserve of a road.
Terminals
Historically, TCB barriers were terminated by being angled down to the ground, and meeting a triangular concrete anchor 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, on higher speed roads, 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, partly to prevent cars riding up the forward barrier and turning over, but also to ensure that full containment is maintained across the ends of each section of barrier.
Open Box Beam (OBB) Barriers
Open Box Beam Barriers consist of an untensioned 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. Current designs are open in cross-section on the side furthest from the traffic, but closed box designs have also been used. In areas where high containment is required, Double Row Open Box Beam (DROBB) will be used, typically on the approach to a bridge pier or gantry base.
Wire Rope Barriers
Another type of barrier 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. Possibly the first use of wire rope barriers in the UK was on the Pennine section of the M62 in 1971. They work because the wire ropes are under tension and wrap around an impacting vehicle to bring it to a halt. The posts are effectively sacrificial and can be replaced.
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
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
There are a number of different temporary vehicle restraint systems available and are usually used in temporary traffic management, typically to protect the workforce or to separate opposing traffic flows in contraflows. Systems include:
Temporary Vertical Concrete Barriers (tVCB)
These are made from interlocking concrete sections which can be forklifted into place. At low speed, they are often laid unconnected to each other, relying solely on friction between the barrier and the surface on which it is placed to absorb any impact. Higher levels of containment can be provided by bolting the barrier sections together and by bolting the barriers to the ground. When correctly installed, they can provide recognised levels of containment similar to those for permanent barrier systems.
The units are directional - they must be installed such that traffic runs adjacent to them in the direction of the arrows marked on the barrier. Tapered terminal sextions are also available but are often ommitted on low-speed roads.
Steel Temporary Barriers
Often known as Varioguard barriers, steel temporary vehicle restraint systems are laid on carriageway and bolted together. They are usually fixed to the ground at each end, and also at intervals along the length of the barrier. They are now more commonly used on high speed roads than temporary concrete barriers.
Mass Barriers
Mass barriers are a temporary vehicle restraint system used in low-speed environments. They have a curved profile on either side and are placed on the carriageway. The curved profile directs an errant vehicle's wheels back towards the carriageway. A fence can be placed on top of the barrier sections and for this reason they are often used to separate high pedestrian flows from adjacent traffic at major events.
Water-filled Barriers
These barriers are not strictly a vehicle restraint system but they are used where there is a need to separate traffic flows on low-speed roads with a barrier more substantial than a line of cones. They are made from interlocking red and white units, some or all of which will be filled with water to keep the barrier in place.
Links
BBC News
- Norfolk bikers fear A11 'cheese-slicer' wire barrier (31 July 2014)
Other
- Motorbike Writer: Are wire rope barriers ‘rider slicers’? (9 April 2014) (archive.org)
- International Business Times: M25 Waltham Abbey Crash - Two men arrested on suspicion of causing death by dangerous driving (10th December 2014)
