Solution Summary: Truck-mounted Attenuator
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). Figure 1 shows the aluminum tubing on the sides and the aluminum honeycomb cartridges in the middle contained in the yellow boxes.
Figure 1. 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. (Source: https://www.traffixdevices.com/products/attenuators/scorpion-tma).
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.
Table 1. Minimum distance chart (source: https://www.dot.ny.gov/divisions/operating/oom/transportation-systems/safety-program-technical-operations/work-zone-control/repository/Shadow_Vehicle_Roll_Ahead_Distance_Table.pdf).
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. Figures 2 demonstrates an example of a TMA with the folding system for storage and transportation.
Figure 2. Folding up process for easier transportation (Safe-Stop 180 TMA folded up). (Source: http://www.energyabsorption.com/products/products_safestop_180.asp)
Risks to workers from being struck by moving traffic are undoubtedly among the most important in the roadside construction zone. Struck-by incidents caused 669 fatalities from work zone crashes in 2014 (https://ops.fhwa.dot.gov/wz/resources/facts_stats/safety.htm). In 65% of work zone roadside fatality transportation events from 2011-2014, a worker was struck by a vehicle in the work zone (https://www.cdc.gov/niosh/topics/highwayworkzones/). In addition, if a vehicle crashes into job a site it will damage construction equipment, causing high repair costs and work schedule postponements. Using TMAs would curb the risk of such accidents.
How Risks are Reduced:
TMAs could signal caution to vehicles passing near a construction work zone. However, the main benefit of them arises when an accident occurs. In that case, TMAs could operate either by steering the collision’s energy and momentum away from the back of the truck on which it is mounted (Figure 3a), or by folding up and collapsing over a short distance and greatly absorbing the collision’s energy (Figure 3b). The device can be used on the move at low speeds (e.g., when workers are picking up roadside temporary traffic control devices) or in a stationary position.
Figure 3. a) Collision energy has been moved away from truck; b) TMA absorbs the collision energy
Other than the workers, using TMAs can reduce the risk of struck by accidents to both truck and traffic vehicle drivers. Figure 4 shows the effect of TMA in this scenario.
Figure 4. Top: Crash with a truck with a truck mounted impact attenuator, bottom: Crash with a truck without an impact attenuator
Effects on Productivity:
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.
Figure 5 shows the connection of TMAs to trailers and Figure 6 introduces examples of Safe Stop Trailers (SST).
Figure 5. An example connection of a TMA. (Source: http://www.energyabsorption.com/products/products_safestop_trailer.asp)
Figure 6. Three examples of SST Trailer TMA:
(b) http://www.energyabsorption.com/; and (c) http://www.highwayguardrail.com/products/tma.html.
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
To obtain information, visit http://cloverleafcorp.com/ or contact 813-649-1336
TrafFix Devices Inc.
To obtain information, visit https://www.traffixdevices.com or contact 949-361-5663
NIOSH Workplace Solutions Sheet
The National Institute of Safety and Health (NIOSH) has published a series of “Workplace Solutions”, which are easy-to-understand recommendations from NIOSH research results. Related to this Construction Solution, please find more information on: Preventing Worker Injuries and Deaths from Backing Construction Vehicles and Equipment at Roadway Construction Worksites
National Trench Safety
To obtain information, visit http://www.ntsafety.com/ or contact 832-200-0988
Structural & Steel Products
To obtain information, visit http://s-steel.com/ or contact 817-332-7417
Energy Absorption Systems, Inc.
To obtain information, visit http://www.energyabsorption.com/index.asp or contact 312-467-6750
Royal Truck & Equipment, Inc.
To obtain information, visit http://www.royaltruckandequipment.com/ or contact 855-202-7129
Main Street Materials, Inc.
To obtain information, visit http://mainstreetmaterials.com/ or contact 888-787-3387
To obtain information, visit http://www.barriersystemsinc.com/ or contact 402-829-6800
Coral Sales Co.
To obtain information, visit http://www.coralsales.com/ or contact 503-655-6351