Types and Functions of Highway Barriers: Factors to Consider
Highway barriers serve to protect motorists from potential vehicle collisions with hazardous objects next to the road, other vehicles, water resources, and roll-over incidents related to slopes and ditches. These are only a few of the risks against which they protect. Read on to discover some types of highway barriers, their characteristics, and their functions.
First and foremost, all types of barriers serve to protect road users and reduce risks while they use the roads. As road safety devices, it is imperative that the use of temporary ones or the installation of permanent types mustn’t increase the risks for road users.
What to consider before installation of permanent highway barriers
For permanent highway barriers, the following must be considered before the installation of road safety devices:
- Number and types of vehicles that daily travel on the path.
- Average speed of the vehicles on a stretch of road.
- Highest allowable speed of travel.
- Ground clearance height of the vehicles.
- Types of roadside hazards and the risk that each pose.
- The distance from the intended installation point and the hazard.
Since any type of permanent fixture also poses a collision risk, it’s essential to only install road safety barriers where really needed. If there is enough space next to the road and before the hazard for a vehicle to come to a controlled stop if it has strayed from the road, then it might be better to do without any type of barrier. Engineer and transportation expert assessments are needed to determine whether installation will enhance road safety.
For a temporary highway barrier installation, it is important to consider the type of risks against which the road users must be protected. If the risk of a collision, damage to property or injury of a worker or road user is eminent without the use of the barrier, then it should be installed. However, high visibility of the devices, proper warnings of the location, and protection of the workers and equipment must still be done.
The clear zone concept is important for both permanent and temporary highway barrier installation decisions. The clear zone is the horizontal clear area needed for recovery from the situation by a motorist as explained earlier.
Where possible, road designs must be adapted to have slopes and hazardous objectives outside the clear zone instead of having to install road safety devices like the W-beam guardrail or a temporary crash barrier. If it’s not possible to make the changes or to design a road accordingly due to the topographical factors present, then the right type of barrier for the particular situation must be considered.
A barrier might be necessary at the following points:
- Bridge – as there cannot be enough clearance for a straying vehicle to return to the safety of the road.
- At the culvert inlets where steep slopes are present.
- Over water resources or directly adjacent to water resources since any straying from the road might prove to be fatal for the vehicle’s occupants.
- On the edge of a mountain pass where a steep cliff is immediately adjacent to the road.
- Near overhead freeway signs, as these signs are vital and motorists must be protected from accidental collisions with them.
- Underneath bridges to protect motorists from direct impact with the concrete support pillars, as such can cause the collapse of a bridge (if hit by a heavy vehicle) and can prove to be fatal for the vehicle’s occupants.
- Between two opposite direction flowing freeways where it is imperative to prevent accidents from spilling over to the other freeway.
- At the on- and off-ramp slopes of freeways to prevent motorists from performing dangerous actions (such as short-cuts over the grass island into fast-moving vehicles).
- Any slopes next to the road that can cause roll-over accidents.
- At tollbooths to prevent vehicles from changing lanes at the last moment, causing a hazard for other road users.
- Crash barriers at tollbooths to protect the operators against out-of-control vehicles.
In the instance of temporary highway barriers, installation might be needed at, for example:
- Excavations next to the road to remove or install culverts or drainage pipes.
- Excavated road surfaces where the road in construction is at a lower level than the highway.
- Where heavy machinery operates.
- Where workers are busy next to or in the road with essential maintenance.
Types of highway barriers
Several types of freeway barriers exist. We’ve already distinguished between permanent and temporary installations. With permanent installations, the location of the installation determines the type of barrier best suited for the situation. Other factors to consider include:
- Stiffness or rigid characteristics of the barrier.
- Deflection capability.
- Prevention of breakthrough.
- Absorption of the kinetic impact energy.
- How much the barrier gives way.
- Redirection capacity of the barrier.
- How the impact energy is dispersed.
- Maintenance needs for the particular type.
- Horizontal footprint of the barrier.
- Repair time and costs involved after every impact.
- Ability to prevent roll-over.
- Ability to prevent breakthrough.
- Ability to keep the colliding vehicle in an upright position.
Let’s discuss each of these factors in more detail in relation to some types of highway barriers to gain a better understanding of the type of permanent barrier to install for a particular situation.
Concrete highway barriers have the most stiffness of all the barriers whereas W-beam guardrails are semi-rigid and wire-rope fences have the least stiffness. The concrete type is the one with the most stopping power and the least chance of a vehicle breaking through. However, it is also the least forgiving and doesn’t absorb any impact energy. The full impact force is taken by the vehicle and its occupants. A head-on collision with such a barrier is the same as hitting a tree or wall.
However, these types don’t require a lot of maintenance and can handle several high-speed impacts. They also don’t have a large horizontal footprint and because the barriers don’t give way to impact, the clearance area behind them doesn’t matter. The blocks are heavy and must be installed with heavy-duty machinery. It is also expensive to transport them.
A characteristic of these is the angled slope at the bottom section of each. This slope makes it possible to redirect a vehicle on side impact, ensuring the vehicle can return to its original travel path. Because breakthrough is prevented and the blocks are high, these can be used where vehicles with high ground clearance travel at high speeds, such as the median of two opposite direction highways.
W-beam steel barrier
Made from hot-dip galvanised steel panels that can be curved to the required angle and fitted to wooden or steel posts attached or buried in the surface, these are known as semi-rigid permanent highway barriers.
The height of the posts is determined by the installation site. If in an urban area where the vehicles travel at slower speeds, the height is normally lower. These types are more forgiving than concrete ones, but because they can give way slightly, a space behind the guardrails must be available for it to happen upon impact from a vehicle.
The W-beam guardrails have a larger horizontal footprint, but also good kinetic impact energy absorption capacity, in addition to medium deflection and redirection ability. The impact energy is absorbed and dispersed through the entire system, thereby taking a lot of the impact force away from the vehicle and its occupants.
The barrier is lower than the concrete type. It is also low in maintenance and a section can be replaced relatively quickly. However, there is the risk of a vehicle vaulting over at high speed if it has enough ground clearance.
Breakthrough is possible, which is something to keep in mind when choosing the installation location.
The individual sections can be curved considerably, making this type of barrier well-suited for installation on sharp turns to protect against collisions with road signs, walls, and trees.
It is also well-suited for median installation on highway grass islands, urban areas and under bridges, at slopes and on mountain passes. These can also prevent accidents from spilling over from one highway to the other. The vehicle can be redirected into its original travel path on side impact. Friction from metal on metal with side impact helps to reduce the vehicle’s speed.
Wire rope fence
It is the least rigid of the three, with the smallest horizontal footprint. It is a suitable alternative to the other two types where there isn’t enough surface space, especially considering that breakthrough is unlikely with the wire rope fence.
With its small horizontal footprint, the wire rope fence with its suspended cables between posts is also the most forgiving on impact. It absorbs a lot of the kinetic impact energy and disperses it through the suspended cables. With such, the impact on the vehicle is low, but it is important to know that an accident from one highway can spill over to the other if a lot of cable movement takes place.
Thus, these types of barriers are suited for installation on highways where travel space is too limited to install a W-beam guardrail or the conventional concrete block. Repairs are usually done quickly.
Different highway barriers exist, each with its specific characteristics that make it suitable for certain applications. It is imperative to consider all the factors mentioned when choosing an appropriate barrier for a particular road section. To this end, it helps to buy from a trusted supplier of road safety products where you can get more information on the best type to install for the particular section and function it must perform.
Reach out to Armco Superlite for more information on the types of highway barriers available, including temporary ones and guidelines on installation.