It is common for work zones to be located near live traffic, which increases the chance of struck-by accidents for construction workers. One solution to reduce this risk is to use Truck Mounted Attenuators (TMAs). TMAs are box-shaped devices attached to the trailer connection of a vehicle; they are engineered and tested to absorb high speed collisions and redirect vehicles away from the workers and machinery in the case of struck-from-behind incidents.

In general, TMAs are recommended for short to medium term construction workers to protect workers on or beside active roads. Some guidlines provide more specific situations for TMAs as follows:

• When the traffic speed is more than 50 mph;
• Road closure is not possible;
• Work is adjacent to or on traffic lane (for both stationary and moving work);
• Work is not protected by safety barriers;
• Work to be done within 'No Go Zone'.

TMAs are collision absorbers attached to a trailer and engineered to withstand increased speed collisions in roadside work zones. They are typically made of an aluminum cage/frame with honeycomb cartridges in the middle. TMAs usually have several stages from one end to the other. Different stages allow for “slower” collisions resulting in less damage and ultimately lower repair costs (e.g., only one stage needs to be repaired instead of the whole attenuator).

This is the Scorpion model. It is made of curved aluminum weather-resistant tubes, with boxes made of an aluminum honeycomb design to also help absorb energy. This product has passed all NCHRP – 350 tests.

The length of a TMA depends on the required impact resistance and the work zone location, as well as the type of TMA used. These vehicles must be placed in consideration of roll-ahead distances after a collision. Table 1 shows the recommended minimum distances for these vehicles from work zones.

The length of the device makes it very difficult to maneuver when fully outstretched. To address this problem, TMAs are usually designed to be folded up for easier transportation.

Using TMAs could ultimately improve productivity. When a collision does occur, TMAs will reduce the risk of equipment damage, so construction work could continue without an unnecessary halt. This also would reduce the number of injured workers and keep the process going. The time and money spent on the device as well as training for TMA operators will be paid off just from injury prevention and a safer work zone. On regular work days (i.e., with no accidents), TMAs could provide a safe environment for workers, allowing them to focus on their tasks and get them done proficiently. 

TMAs require a trained professional to operate, set up, maintain, and transport the equipment. Improper use might minimize safety outcomes for the work zone. One should remember that TMAs cannot operate alone; proper signage must be utilized in accordance with the needs of the work zone. A four-hour training course provided by the American Traffic Safety Services Association (ATSSA) will train people on the design, maintenance, monitoring, installation, and removal of TMAs for all temporary road/highway work zones. Upon completion of the course, the operator will be certified to use TMAs in the work zone. For more information see: http://www.atssa.com/TrainingCertification/CourseInformation/TruckMountedAttenuatorsTMA.aspx. 

The National Cooperative Highway Research Program (NCHRP) has tested vehicles of different mass, angles of collisions, and speeds against TMAs. These tests resulted in several suggested requirements that are reported in NCHRP report 350 (http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_350-a.pdf). TMA manufacturers are required to review these details and fulfill the requirements needed to pass these tests. 

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