Crash cushions are an example of road restraint products with many applications in both permanent and work-zone locations. Permanent systems are put in places like gore areas on highways or bridges and are expected to last for at least 10 years. Temporary systems such as those in construction work zones are typically placed at temporary concrete barrier ends to minimize the impact of a collision by absorbing its energy. While many manufacturers have designed various types of cushions with different materials and details, each system usually consists of crushable, energy-absorbing cartridges within a framework that holds them together.

It is common for work zones to be located near live traffic, which increases the chance of struck-by accidents for construction workers. Temporary concrete barrier walls are among suggested solutions to reduce this risk. While these barriers help protect workers from vehicle collisions, their rigid nature could create severe outcomes for workers (e.g., a vehicle rollover into the work zone) and for drivers (e.g., injuries resulting from the high kinetic energy of a collision). To reduce the impact of collisions, temporary crash cushion systems can be placed in front of concrete barriers as depicted in Figure 1. Two common types of crash cushions are called gating and non-gating systems. This classification is based on the clear area behind the crash cushions. A clear-zone is defined as an area without any trees, cliffs etc. Non-gating (Figure 1a) systems are preferable in work zones, especially when used with concrete barriers as they need less of a clear-zone behind them. However, in cases with more clear-area behind the barricades, gating systems are recommended as they are more economical with their simpler design and production (Figure 1b). 

(Photo courtesy of Lindsay Transportation Solutions)

Figure 1 – Installing crash cushions before concrete barriers – non-gating (top), gating (bottom)

Figure 2 shows the two types of crash cushions in NCHRP 350 Test. It's apparent that non-gating cushions provide enough strength to stop the cars before their intrusion into the work-zones. In the case of head-on accidents, the system can stop the vehicle by crushing and ripping cartridges that are designed inside the system. One should note that each of these systems have several types based on their design and the material.

Figure 2- non-gating (top) crash cushions with the sufficient structure need much less clear area than gating (bottom) products. 

Another important feature in selecting the right type of crash cushion is its ability to deflect vehicles back into the road. Based on this feature, two types of cushions have been designed: redirective and non-redirective. This feature is particularly important in side-impact crashes. Redirective systems act like guardrails by absorbing some energy and deflecting the vehicle back to the roadway. Repairing redirective products is costly, and more frequently the products must be replaced. Non-redirective cushions, on the other hand, are not capable of deflecting a side impact but replacing damaged units could be done quickly with lower costs. Most of the products in the market are non-gating and re-directive as they can provide higher safety performance. One may note that it’s important to use speed reduction signs near work-zones in combination with non-redirective products, as these solutions are not designed to bear high-speed collisions.

Table 1 introduces examples of crash cushion solutions that are specifically designed and manufactured for construction work zones. Note that while the mentioned features of cushions (gating vs. non-gating and directive vs. non-directive) explain two different aspects of these products, usually directive products are also non-gating and therefore can protect areas closer to the cushions. On the other hand, non-redirective products are also gating and therefore are appropriate only for large work zones where workers are not close to crash cushions or in areas in which vehicle speeds are relatively low.

Table 1 – Crash cushion systems for construction work-zones

Selection of the right system for each work zone depends on its unique situation. Here are seven variables to consider before choosing a system.

· Distance from cushion to traffic. For distances less than 8 feet, choose redirective products; otherwise non-redirective products would work properly in most cases.

· Traffic volume. Use redirective products in projects with higher volume traffic.

· Duration of use.  Longer projects need redirective systems. Non-redirective cushions are not appropriate for winter as their performance could be affected in freezing temperatures.

· Type of surface. Non-redirective products can be placed on any surface, while redirective ones generally need a concrete pad.

· Size of concrete barrier. Protection of wider barriers is easier with non-redirective solutions.

· Size of cushion. Redirective products need less space than the other type.

· Clear space behind. If there is no clear space behind cushions, only redirective systems are allowed to be installed.

Crash cushion systems have no direct effect on productivity as they do not change the way workers complete their tasks. But selecting the right types of cushions can save repair time and improve long-range productivity. Reducing the severity of accidents could also reduce the distractions for workers and enhance workers’ productivity.  

These design considerations should be taken into account when using crash cushions:

• Non-redirective cushions could be installed on concrete or dirt;
• Consider non-redirective products as an array of sand filled barrels;
• If barrels are located behind a curb, the curb should be grounded in areas close to the barrels;
• The weight of the barrels should increase such that the heaviest barrel is placed at the rear;
• Be aware that the barrels don’t pose a risk to opposing traffic. (the situation depicted in Figure 4 must be avoided);
• Redirective cushions must be installed only on a flat, paved surface;
• If a curb is present, it must be grounded to the level of the cushion’s foundation, or vice versa;
• One other feature that is also important, particularly for temporary usage of cushions, is their installation requirements. A quick installation process could be a very significant advantage for these products.

Figure 4 – The cushions may present the risk of collision between opposing vehicles and the heaviest barrel. The conditions presents here should be modified.

Behzad Esmaeili, Ph.D. - University of Nebraska- Lincoln
Pouya Gholizadeh - University of Nebraska- Lincoln
Sogand Hasanzadeh - University of Nebraska- Lincoln
Erik Bruening - University of Nebraska- Lincoln
Brett Farquhar - University of Nebraska- Lincoln