A guardrail is a type of traffic barrier. It is often referred to as an Armco or W-beam barrier. In countries like the USA and South Africa, the term “highway guardrail” is often used to refer to the W-beam or three beam barrier on the side of the road.
What is the purpose of the guardrail?
It is a type of crash barrier in the sense that it prevents straying vehicles from leaving the safety of the road and to prevent these vehicles from hitting unforgiving obstacles with the likes of trees, signposts, rock faces, abutments or walls. With this explanation in mind, the main purpose is to improve road safety.
How does the guardrail improve road safety if it is also a hazard?
True, it is a barrier. Therefore, it is an obstacle just as much as a wall or a tree is a hazard. However, it is more forgiving than an object that cannot give way a bit, redirect, absorb kinetic impact energy, and disperse part of the collision energy across the system.
Guardrails improve road safety conditions in several ways:
- Protect pedestrians in vulnerable spaces like those areas around nursery schools, primary schools, retirement homes, and hospitals by creating a barrier between the drive and walk areas.
- Where installed in the middle or on the island between two opposite flowing lanes of a highway, these are called median barriers. They have beams on both sides and serve to prevent cross-over between the two opposite flowing lanes. Such cross-over can cause a huge pile-up accident with potential fatalities. In this sense, the guardrail serves to hinder access to a road, prevent an accident from one freeway from spilling over to the opposite direction freeway, and to minimise the risk of head-on collision due to illegal U-turns on freeways.
- Protect vehicles from the impact severity associated with hitting a static object, such as a tree, signpost or wall by creating a softer, more forgiving barrier that reduces the impact of the accident by absorbing a large part of the kinetic energy, and dispersing it throughout the system. Instead of the vehicle and its occupants taking the full impact, the guardrail takes a large part. Although it cannot guarantee any potential fatalities or injuries from occurring, the severity of the impact is significantly less.
- Improves driver confidence, especially at night when it is difficult to see the exact edge of a road. When motorists drive on a carriageway bordered by a steep slope or cliff, they tend to keep closer to the median line. This tendency is dangerous as it can lead to head-on collisions. With a visible and strong barrier to the left edge, the motorist has more confidence to keep left, thereby reducing the risk of head-on collisions.
- Guardrails also serve to redirect straying vehicles back to their original safe path upon angled-side impact. The friction of the barrier steel and the side colliding vehicle helps to reduce the speed of the vehicle. It also causes a loud sound, which helps to wake up a motorist if the person has fallen asleep.
If guardrails are intended to improve road safety, why do motorists often incur injuries when their vehicles hit these barriers?
Keep in mind that these are barriers and, therefore, obstacles. These barriers are tested to handle collision impact from a vehicle driving at a specific speed. The angle of impact, whether the vehicle has mounted the rail or because of its high ground clearance, has gone over it, and the speed of travel affect the outcome of the accident. Since no two accidents are exactly the same, it is impossible to predict 100% protection. The test is rather if the severity of the accident, damage to the vehicle and its occupants would have been worse if there hadn’t been a barrier in place.
Engineers and road designers carefully consider installation of guardrails. For one, they have to determine whether or not there is enough space next to the road to enable the safe return of a vehicle to the road or safe stopping before it hits a hazard or enters a potentially fatal situation. This is called the clear zone. If the vehicle cannot return safely to the road through corrective action from the driver without hitting a hazardous object or ending up in a potentially fatal situation, then it is better to install the guardrail. In this sense, the barrier serves as protection when the clear zone is not sufficient to improve road safety.
These engineers consider the appropriate height requirement according to the average speed of travel on the road and the types of vehicles using the road. Guardrails in urban areas are generally lower because the vehicles travel at slower speeds, unless on the highways where higher speed limits are in place. The main point here is whether to protect pedestrians and cyclists against straying vehicles or to shield motorists from the severity of impact with unforgiving objects like tunnel walls.
Common sites for installation of these barriers include:
- Bridges over carriageways, cliffs and water sources.
- At severe curves in the road.
- Near slopes close to the road edge.
- At culverts or any areas posing a danger related to the severity of the drop.
- Near large signposts or other elements, which can be hazardous.
- Next to the walkways close to schools and retirement homes.
- Next to the freeway to prevent pedestrians from crossing and to provide them protection against straying vehicles.
- In the median of two opposite flowing highways.
How W-beam guardrails differ from other types of road barriers
Several barrier types exist. They are categorised according to their functions and the rigidity of the structures. Although a W-beam guardrail can be installed in many places, it may not always be the best type of barrier for the situation.
Where the road surface is limited or where the drive lane width doesn’t allow for the installation of the W-beam types, the wire-rope fence is an option. This type has cables between posts instead of a steel beam attached to wooden or metal posts. It has a smaller horizontal footprint. It has less redirection capacity, but has good energy-absorbing characteristics.
The concrete barrier is the most rigid and unforgiving. However, where it is essential to prevent breakthrough or a vehicle from going over the barrier, then this type is installed.
Where to get more information regarding available W-beam guardrails in South Africa