Roadside Safety - Barriers
Safety barriers are used to stop ‘out of control’ vehicles from:
- leaving the road and hitting roadside hazards, including slopes (roadside barriers)
- crossing into the path of on-coming vehicles (see median barriers).
They are designed to redirect the vehicle and have a lower severity than the roadside hazard they protect. There are several types of safety barrier (but within these types there are different systems which have their own specific performance characteristics).
Flexible barriers are made from wire rope supported between frangible posts. Flexible barriers may be the best option for minimizing injuries to vehicle occupants, however they may pose a risk to motorcyclists. These barriers deflect more than other barrier types and need to be repaired following impact to maintain their re-directive capability.
Semi-rigid barriers are usually made from steel beams or rails. These deflect less than flexible barriers and so they can be located closer to the hazard when space is limited. Depending on the impact these barriers may be able to redirect secondary impacts.
Rigid barriers are usually made of concrete and do not deflect. Rigid barriers should be used only where there is no room for deflection of a semi-rigid or flexible barrier. Rigid barriers are often utilized at high volume roadwork sites to protect road workers or other road users particularly where another barrier type is awaiting repair. Currently (depending on their height and other details) these provide the highest level of containment of heavy vehicles. In most cases following impact these barriers require little or no maintenance.
Vehicle parapets are rigid barriers in steel, concrete or a combination of both. They are installed onto highway structures such as bridges, retaining walls or building facilities. Their main purpose is to restrain an errant vehicle from falling off the structures.
Much of the benefit from the use of barriers comes from a reduction in crash severity. Although a crash may still occur, it is likely to have a safer consequence than colliding with the object that the barrier is protecting.
On existing roads retrofit of safety barriers can be challenging due to incompatibility between road layout, ground condition, roadside space, etc. and technical criteria for the installation of safety barriers. In these circumstances, solutions are often context-sensitive and may need to be tailored-made. Solutions for existing roads should be formulated with on-site surveys, comparison of options and risk assessments.
In dense urban areas where the roadside is needed for crossing, access to vehicles and loading or unloading activities, bollards may be a means to protect pedestrians from errant vehicles. However, rigid bollards may not be forgiving at high speed except for products designed to concurrently attenuate an impact.
Barrier end terminals
The need for end terminals arises from the deployment of safety barriers. Collision with untreated end terminals can result in lead to violent deceleration or penetration of the barrier into the compartment. On the other hand, a sloping barrier end can launch an errant vehicle into the air followed by rollover, falling off a drop or collision with rigid objects. All these scenarios have high potential for severe injuries to occupants of an errant vehicle.
End terminals of safety barriers should be addressed proactively on a route-wide basis.
The first step is to minimise the number of end terminals or relocate them to safer positions:
- Closing short gaps between two sections of the same safety barriers.
- Closing short gaps between two sections of different safety barriers with a transition.
- Extending the safety barrier upstream to locations with lower traffic speeds e.g. start of slip roads.
- Extending the safety barrier upstream of curves and other vulnerable sites.
The remaining sites should be addressed by the following measures:
- Extending and flaring the safety barrier to blend in with an upstream slope or anchor to an abutment wall.
- Adopting crash cushions or end terminals.
If these are not possible, the following treatments may be considered provided that traffic speed is low to moderate:
Flaring of beam barriers so that the end terminal is as far as practical from the general barrier alignment
Curved-out layout of rigid barriers or masonry parapets at a maximum angle of 40 degrees from the general barrier alignment
Sloping end of W-beam barriers of gentle gradient anchored into the ground may be an acceptable treatment. At low to moderate speed, an errant vehicle may ride over the barrier. There should not be major roadside hazards which can be reached by the errant vehicle.
Openings are sometimes needed along a safety barrier for emergency refuge, evacuation, operation or maintenance. These openings may be provided by an overlapping barrier layout such that the leading end terminal of the second barrier is shielded by the trailing section of the upstream barrier. However, this arrangement may not be suitable for undivided road where an errant vehicle may encroach onto the roadside from the opposite direction.
- If properly designed, installed and maintained, barriers should reduce the severity of crashes involving ‘out of control’ vehicles.
- When used in the median, safety barriers can greatly reduce the likelihood of head-on crashes.