In this article, Civil Engineering expert, Kevin Gorman, P.E., C.C.M. discusses temporary traffic impact attenuators. His discussion includes an overview of the different types of attenuators, and an explanation of the standards and best practices in their selection, site planning, usage, and maintenance.
Temporary Traffic Impact Attenuators
Impact attenuators are a common sight across the nation’s highways. They are installed to offer additional protection to structures, motor vehicles, their occupants, as well as highway construction workers. The 2012 Manual on Uniform Traffic Control Devices (MUTCD) defines impact attenuators as “systems that mitigate the effects of errant vehicles that strike hazards, either by smoothly decelerating the vehicle to a stop when hit head-on, or by redirecting the errant vehicle. Impact attenuators in temporary traffic control zones protect the motorists from the exposed ends of barriers, fixed objects, and other hazards.”
The mitigation is accomplished by the attenuator absorbing some of the motor vehicle’s kinetic energy (i.e. slows the vehicle) when the vehicle strikes the attenuator. After initial impact, some attenuators will redirect the motor vehicle away from the hazard or object, be it a bridge pier, piece of construction equipment, or highway workers. You will see attenuators placed in front of fixed hazards along highways as either permanent set-ups, or as temporary attenuators used during a highway construction project.
Impact attenuators are tested and classified by “test-level” according to the AASHTO Manual for Assessing Safety Hardware (MASH) which replaced FHA NCHRP Report 350. The classification is based on the maximum speed of a vehicle during a collision for which the attenuator is designed. During design of Traffic Control Plans for construction, designers will specify the appropriate test level for specified impact attenuators in consideration of the vehicle speeds along the highway upon which the attenuator will be set.
Temporary traffic impact attenuators (TIAs) can be set in place for long durations, or set up daily to “follow” a work crew as they progress down the highway with their work. Aside from the duration during which an attenuator is planned to be set at a particular location, the functionality of temporary and permanent impact attenuators are the same. This discussion will focus on the placement, monitoring, and safety measures involved in their use.
You will find temporary traffic attenuators placed within work zones to protect various features of the Traffic Control Plan (TCP). Attenuators will be placed at the start of temporary concrete barrier and at construction area openings to protect drivers from impacting a blunt end. They can be placed to protect bridge piers, or columns that support roadway signs. The MUTCD serves as the basis for project specific Traffic Control Plans, and along with applicable state regulations, serves as the standard of care even on projects with no formal TCP. See MUTCD example figure “Typical Application 34.” Prior to placement, the project needs to verify that the materials used have the appropriate certifications and approvals for use, as well being the correct test-level with FHWA acceptance. The project should also have a copy of the installation and instruction manual from the attenuator manufacturer.
During placement, appropriate temporary traffic control measures should be enacted to direct motorists around the installation area. Once it is installed and proper installation verified, monitoring is the next step for a long-term set up.
Inspections should be performed and documented twice a day to ensure the attenuator has not been damaged. If an attenuator is found to be damaged, prompt replacement or repair is required. Should the project see a pattern of repeated collisions with an attenuator, a re-evaluation of its placement should be made with consideration of moving or adjusting the attenuator’s location to reduce the number of vehicle impacts.
A traffic impact attenuator can actually become a safety hazard in its own right if it is installed improperly, if non-approved systems are used, or if it is not maintained and repaired regularly. For example:
- Non-approved attenuators can malfunction, resulting in (for example) the guide rail piercing the vehicle and injuring the occupants, instead of absorbing the kinetic energy and slowing the vehicle as they are designed to do.
- Attenuators placed improperly relative to the traffic they are protecting can result in unintended vehicle strikes.
- Use of the wrong classification of attenuator can result in injuries that could have otherwise been avoided.
The highway engineers at Robson Forensic can help you determine whether or not a temporary traffic impact attenuator was dangerous in a manner that was a cause of the incident you are investigating. When appropriate, and with client approval, we can involve other experts such as: vehicle engineers, crash reconstructionists, commercial vehicle specialists, meteorologist, human factors, and lighting experts.
Federal Highway Administration. (2012). Manual on Uniform Traffic Control Devices. FHWA.
Transport Infrastructure Branch. (n.d.). Road Safety Barriers Design Guide Part B. Department of Infrastructure Energy and Resources.
Highway Engineering Investigations
The municipal and highway engineers at Robson Forensic are frequently retained to determine if the design, construction, or maintenance operations of roadway systems contributed to the cause of motor vehicle collisions, pedestrian strikes, or other injuries.
For more information submit an inquiry or contact the author of this article.
Civil Engineer & Highway Engineering Expert
Kevin Gorman is a Civil Engineer with nearly 20 years of professional experience involving roadway, infrastructure, and heavy construction. His background includes the construction, inspection, maintenance, and failure analysis of Federal, State, and Municipal projects from the perspectives of an engineering consultant, State DOT engineer, and construction manager. Kevin applies his expertise to forensic casework involving injuries and financial claims related to the performance and construction of transportation and other infrastructure systems. He earned his bachelor of science in Civil Engineering from Cornell University and later earned his Master of Science, with a focus on Criminal Justice from Nova Southeastern University. Kevin is a member of the Construction Management Association of America (CMAA), and in 2009 earned CMAA’s certification as a Certified Construction Manager (CCM).