As you drive down any portion of our interstate highway system, you may not be aware that there are safety features besides the ones in your automobile, such as seat belts and air bags, that are protecting your life in the event of a crash. Some are obvious, such as the very nature of the highway being divided or having controlled access via entrance and exit ramps. These features not only reduce the chance of life threatening crashes occurring, but also mitigate the consequences of those crashes that do occur by separating oncoming traffic.
But what about other types of crashes, particularly those that do not involve other vehicles but rather crashes into highway structures themselves? Safety features such as guardrails and concrete barriers protect many of these situations. However, even these safety features themselves—designed to redirect errant cars safely back onto the road—can be life-threatening in their own right if impacted directly or, particularly, on less-forgiving end conditions. In many of these cases, impact attenuators provide additional safety.
Attenuators and highway safety
Impact attenuators are typically used to shield rigid roadside objects—such as concrete barrier ends, steel guardrail ends and bridge pillars—from oncoming automobiles. They are designed to meet federal guidelines to bring a wayward vehicle to a safe stop or redirect the vehicle away from the rigid object. By one manufacturer’s estimate, impact attenuators have saved more lives worldwide than seat belts and air bags combined.
What contributes to such dramatic effectiveness in highway safety? Clearly, it is extensive product development and testing. Impact attenuators have gone through dramatic engineering changes in just a few years as more effective and lower maintenance designs are developed. An earlier generation attenuator, such as a hydrocell, was comprised of several sealed pockets of liquid (usually antifreeze for colder climates) and had a very high maintenance cost. Each time the attenuator was impacted, the DOT had to examine which cells had burst upon impact, replace them and refill them. Additionally, there was an adverse environmental impact with the antifreeze contaminating nearby soils. Subsequent models, such as hex-foam attenuators, got rid of the liquid component and relied instead on a network of hexagonal cells to compress and absorb energy. However, depending on the degree of impact, the maintenance and repair of the hex-foam attenuator usually required replacing the entire unit at a substantial cost.
Currently, the state-of-the-art attenuator is called a quadcell. This attenuator is designed with an accordion-type frame of steel diaphragms and fender panels that collapse on impact. Within this steel framework are foam cartridges that help absorb the energy of the impact. Not only is this attenuator the most effective at absorbing impact energy and increasing occupant safety, the interchangeable components are more easily replaced and consequently reduce DOT parts inventories. Unless the entire steel frame is damaged, the DOT only needs to repair or replace damaged cartridges or broken steel diaphragms.
Equally important as the giant strides made in design is how these products are used on new or improved roadway designs. Ultimately, it is the traffic engineering consultant who integrates a well-designed and crash-tested impact attenuator into a safe highway condition. Some of the most important guidelines followed in this process are those established by the Federal Highway Administration (FHWA).
Federal regulations play an important role in recommending which attenuators are candidates for various applications. Vehicle weights and speeds, impact angles, stopping distances and elevation changes are some of the factors used to define attenuator types for various road conditions. These guidelines are then used by DOTs and traffic engineering consultants to select the appropriate product for the right application.
The FHWA establishes the criteria whereby equipment can be properly evaluated and installed. In 1997, after more than 10 years of reviewing and evaluating the results of the National Cooperative Highway Research Program (NCHRP), FHWA formally approved two of the NCHRP’s reports and incorporated them into its federal highway safety criteria. Now, all safety devices installed on the National Highway System (NHS) must be crash tested and found to comply with the recommendations of these reports, entitled Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances—Report 230—and Recommended Procedures for the Safety Performance Evaluation of Highway Features—or Report 350.
It is important to know that these criteria apply regardless of whether a project is being built by local, county, state or federal government and are not dependent on the funding source. In other words, the safety criteria for safety devices such as impact attenuators are not only mandated, but also are applied universally across the entire interstate highway system. At the same time, however, each state may have its own additional criteria to add. Then the traffic engineer makes a final determination on which model to select for a particular application and how to design it into the specific site in question to maximize highway safety.
Putting the pieces together
The traffic engineer’s role is crucial in realizing the potential safety of devices such as impact attenuators. Like any project, a highway project involves clients, manufacturers, suppliers and consultants who are designing for unique and specific site requirements. Therefore, it is the traffic engineering consultant’s responsibility to collaborate with other members of the project team, interpret the applicable highway safety guidelines, obtain current product information from manufacturers and suppliers and respond to parameters defined by the site to arrive at an optimal design solution using the proper attenuating device.
The highway design process can be demonstrated by examining a renovation of two portions of I-71 in Cincinnati for the Ohio Department of Transportation (ODOT). These projects included the specification and location designs of 11 impact attenuators. The design and review process played an important role in selecting the right attenuator products for the job and installing them in an appropriate manner to maximize safety.
The first step in the design process involved the traffic engineering consultant, Woolpert LLP of Cincinnati, Ohio, obtaining current ODOT regulation updates to the federal NCHRP-350 standard. The consultant also worked closely with ODOT’s traffic and maintenance engineers to ensure that the impact attenuators being considered for the project were state-of-the-art, low-maintenance and compatible with the client’s overall management and maintenance programs.
At the same time, the traffic engineering consultant worked with the product manufacturer and provided project specifications for their review. Together, the consultant and manufacturer were able to determine the best devices for the I-71 application. After the initial selection, the consultant returned to ODOT for a design review. Design changes were made based on ODOT’s suggestions and then a final review and approval cleared the way for product pricing, ordering and installation.
This process for the I-71 rehabilitation projects resulted in several advantages for ODOT:
Impact attenuators were selected to meet not only the federal NCHRP criteria, but also current ODOT guidelines and manufacturer recommendations.
The consultant included structural and transition details for the specific attenuator sites in the construction documents, eliminating the need for ODOT or the contractor to determine these details on-site at the time of installation.
Close collaboration among the consultant, manufacturer and contractor allowed for expedient product ordering and procurement. The resulting on-time delivery kept the project on schedule.
The new impact attenuators for I-71 have resulted in a safer highway with lower maintenance costs.
Safety through design
Impact attenuators are intended to enhance the safety of our nation’s high-speed divided highways. Like any product, however, choosing the right alternative and using it properly is essential to realizing its full potential. The traffic engineer provides safety through design by interpreting impact attenuator guidelines, selecting products and detailing construction documents. Designing impact attenuator installations with specific site and traffic conditions in mind adds value to a highway project with increased safety and reduced maintenance.
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