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Crash cushions: the invisible guardians of our roads

Uncategorized
March 22, 2024 8 min to read

Road accidents can have a devastating impact on people’s lives and road safety. To address this challenge, crash cushions represent a fundamental solution, but in some cases, these devices can pose a danger by being fixed obstacles themselves.

 

In this article, we will explore in detail the world of crash cushions, what they are, and how they function according to the European and American regulations that govern them. We will also analyze their performance characteristics, the differences between redirective and non-redirective attenuators, the repairability of the devices, and the contexts in which they are used to ensure road safety in order to select the most suitable crash cushion.

What is a crash cushion?

The mission of passive road safety is to prevent a driving error from turning into a tragedy. A crash cushion is a passive road safety device designed to minimize damage and injuries during a road accident. The crash cushion is positioned along roads and highways, ready to absorb the kinetic energy of vehicles in a controlled manner, minimizing the impact and protecting both people and vehicles involved.
The function of an attenuator is to:

 

  • Absorb the impact of vehicles up to a speed of 130km/h (the most common speed limit on highways worldwide);

 

  • Redirect the errant vehicle back onto the road, avoiding rollovers, twists, head-to-head collisions, or others.

How do crash cushions work?

Generally, when a vehicle collides with a crash cushion, it deforms gradually, dissipating the vehicle’s kinetic energy and reducing the impact acceleration. This process is crucial for mitigating damage and protecting people’s lives.

IMAGE 02 – crash cushions

European Standard EN1317 and American Standard MASH

Road safety is a universal priority, and to ensure it, there are regulations that define the performance criteria of passive road safety devices. Specifically, in Europe, the reference standard is called EN1317 and is widely used in non-European countries such as the United States, Canada, Mexico, Australia, and New Zealand. In these countries, the performance criteria are evaluated in accordance with the Manual for the Evaluation of Safety Devices (MASH).

IMAGE 03 –Standards

These standards establish criteria for impact tests, evaluation, and certification of attenuators, ensuring that they meet road safety standards. EN1317 standard sets the criteria for testing crash cushions according to different approaches, four speed classes (50, 80, 100, 110), and with vehicles of three different masses (from 900kg to 1500kg).

MASH standard
MASH crashtest

MASH standard differs not only in the different test angles but also in the types of vehicles used: a 1100 kg car and a 2270kg pickup truck, meeting the safety needs represented by heavier vehicles, which constitute an increasingly significant percentage of the circulating fleet.


In light of this increase in vehicle size and mass, such as SMA, we posed the following question: what behavior do crash cushions tested with a 1500kg vehicle have in the event of a collision with a heavier vehicle?

Performance of crash cushions

Crash cushions are evaluated based on their behavior following a collision, including their ability to absorb kinetic energy, redirect vehicles, and lateral displacement. Additionally, they must ensure safety for both vehicle occupants and all road users. Part 3 of the EN1317 standard regulates performance classes, acceptance criteria for impact tests, and test methods for attenuators. Compliance with these criteria allows for the issuance of the CE marking. The following parameters are examined:

 

  •  Acceleration Severity Index (ASI)
  • Redirect Zone
  • Permanent lateral displacement
The ASI (Acceleration Severity Index)

 

is one of the most indicative parameters regarding the severity of the impact suffered following the vehicle’s collision with the device. Measurements are made using accelerometers on the vehicle and compared to parameters considered “limit,” below which the risk to passengers wearing seat belts is very low.

Acceleration Severity Index

Naturally, the criterion used for ASI provides useful indication of the risk to the vehicle’s passengers in the event of a collision; however, it is not the only criterion for demonstrating passenger safety. A more effective parameter for measurement is the calculation of biomechanical parameters. The redirect zone (whose result is indicated by the letter “Z”) is an index that measures the device’s ability to redirect the vehicle. A positive result is determined by a deceleration of the vehicle and reduced distance from the impact point.

Redirect zone

Finally, permanent lateral displacement (indicated by “D”) denotes the displacement of the device from its axis or the loss of fragments from the attenuator following the impact. It is very important for this index to be in D1 because a higher value would imply that the attenuator could invade the roadway and cause further accidents.

Permanent lateral displacement

The difference between redirective and non-redirective crash cushions

A distinctive feature of impact attenuators is their ability to redirect vehicles in the event of a collision. The crash cushion can be:

 

  • Redirective: tested to absorb side impacts as well and thus with the ability to redirect the vehicle along its original path.

 

  • Non-redirective: tested only for frontal impacts with a high risk of unpredictable performance in the event of side impacts. For non-redirective attenuators, only a frontal test is provided. Therefore, we do not know how the device would behave in the event of a side impact. This aspect must always be considered when selecting the attenuator to install.

Is the crash cushion repairable?


A classification of crash cushions distinguishes between sacrificial and repairable products. The use of sacrificial attenuators involves disposing of the device after impact. Repairability is a crucial aspect of crash cushions. It should be possible to repair or replace damaged components easily after an accident, reducing maintenance costs and ensuring continuous protection for road users.

Selection and application of a crash cushion

How to select a crash cushion?


When selecting the device to install, the context must be considered, which is determined by a series of factors such as:

  1.  Speed limit;
  2. Shape of the obstacle;
  3. Width of the obstacle;
  4. Ground conditions;
  5. Other elements regarding the surrounding environment.

Often, on roads, we still find attenuators made of plastic that are highly risky, especially if installed inside tunnels, as they contribute to the spread of flames in the event of a fire.

PLASTIC

Not only the material problem but also the road obstruction and anchoring support influence the correct selection of the product. This is why each problem corresponds to a specific solution.

In which contexts are crash cushions applied?

 

Crash cushions are used in various contexts: The most common context is certainly the high-speed road (highway, expressway), as well as main and secondary rural roads. However, there are special contexts where crash cushions are used to prevent even moderate-speed accidents from turning into tragedies.

 

  • Work areas: in road work zones, the attenuator protects workers and motorists during road construction or maintenance. It is applied at the end of the concrete barrier that delimits the work area from the roadway. In this way, the attenuator prevents head-on collisions with the rigid barrier element, as well as the vehicle’s access to the confined area of work, avoiding a possible collision with the operators.
Road work zones
  • Urban Areas: in cities, the crash cushion is installed to protect critical points such as trees, traffic signal poles, pillars, and portals. The device also becomes functional in ensuring the safety of vulnerable users, being equipped with a tested covering for the protection of motorcyclists.
Road work zones
  • Tunnels: the crash cushions installed in a tunnel prevents serious accidents against the wall of the niche or the tunnel bypass. This is a special context where the size of the device and the materials used, combined with a minimally invasive anchoring procedure and the guarantee of high performance, really make a difference
Road work zones

What is the safest crash cushion in the world?

Among the attenuators installed on Italian and worldwide roads, the SMA device that complies with European regulations is named Leonidas and represents the pinnacle of road safety. Characterized by a compact length (5840 mm in the 110 class), made entirely of steel, with redirective capacity, and CE certified for asphalt use, Leonidas fits the specific needs of the road and the surrounding environment, contributing to making roads safer for everyone.

IMAGE 9 – ANDROMEDA BRIDGE RAIL H4

It is the only attenuator tested according to two different standards – European EN 1317 and American NCHRP 350 – which sets it apart from other attenuators: it allows demonstrating how the system works in the event of impact with both light vehicles – as required by European standards – and heavy vehicles (such as American pickups).

 

To recap, these are the reasons that make Leonidas the safest crash cushion in the world:

 

1. CE certified according to EN 1317 standard
2. Redirective
3. Fully made of steel
4. Repairable
5. Tested with a 2000 kg pickup truck
6. Tested at 130 km/h

 

But above all, Leonidas represents…

…The most compact crash cushion in the world!

 

The model for the 110km/h speed class is less than 6 meters long.
Comparing it to other competitors, the difference – in terms of length for the 110 class – is at least 2.5 meters. This is a uniquely advantageous feature in terms of safety. In fact, a “compact” crash cushion reduces road invasiveness and allows for greater braking space on the road to avoid further accidents: in 2.5 meters, you can slow down enough to steer before impact.

Hence the motto:
shorter means safer.

To discover the SMA impact attenuator family,

Click here

CONCLUSION


In conclusion, crash cushions are fundamental devices to ensure road safety and protect human lives. Compliance with regulations and the adoption of best practices in the design and installation of attenuators are crucial to ensuring a safer road environment for everyone. Leonidas by SMA is the best example of how technological innovation serves road safety, bringing vehicle and passenger protection to a higher level.

 

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