Protection Vehicles - Positive Protection for Work Zone Personnel
Source: Frank Brewer
We who are involved in Work Zone design understand Jerry Ullman's statement. Therefore, we must protect Work Zone workers from avoidable and unavoidable hazards! The Mobile Work Zone, by its definition, presents a most difficult situation for providing worker safety. How to protect workers without a static protected work area? One type of positive protection developed and often used is the Truck-Mounted Attenuator (TMA). The TMA is positioned as a shadow vehicle, relative to the workers, work vehicles or the immediate workspace. Properly used, the TMA (trailed behind the work vehicle and potential workers on foot) is to be sacrificed in the name of protection. Even when used properly in mobile operations, drivers of the TMA are placed at high risk for injury.
Table 1. Recommendations for the Assignment of Shadow Vehicles | ||||
|---|---|---|---|---|
| Closure/Exposure Condition | Freeway | Non-Freeway with Speed Limit | ||
| >=50 mph | 40-45 mph | <=35 mph | ||
| Shadow vehicle for no formal lane closure for operation involving exposed personnel | Very highly recommended | Very highly recommended | Very highly recommended | Very highly recommended |
| Shadow vehicle for no formal lane closure for operation NOT involving exposed personnel | May be justified* | May be justified* | May be justified* | May be justified* |
| Shadow vehicle for no formal shoulder closure for operation involving exposed personnel | Highly recommended | Highly recommended | Recommended | Recommended |
| Shadow vehicle for no formal shoulder closure for operation NOT involving exposed personnel | May be justified* | May be justified* | May be justified* | May be justified* |
| *May be justified on the basis of special conditions encountered on an individual project. | ||||
Table 2. Recommendations for the Application of Truck-Mounted Attenuators | ||||
|---|---|---|---|---|
| Closure/Exposure Condition | Freeway | Non-Freeway with Speed Limit | ||
| >=50 mph | 40-45 mph | <=35 mph | ||
| Shadow vehicle for no formal lane closure for operation involving exposed personnel | Very highly recommended | Highly recommended | Recommended | Desirable |
| Shadow vehicle for no formal lane closure for operation NOT involving exposed personnel | Highly recommended | Highly recommended | Recommended | Desirable |
| Shadow vehicle for no formal shoulder closure for operation involving exposed personnel | Highly recommended | Recommended | Recommended | Recommended |
| Shadow vehicle for no formal shoulder closure for operation NOT involving exposed personnel | May be justified* | Recommended | Desirable | May be justified* |
| *May be justified on the basis of special conditions encountered on an individual project. | ||||
Table 3. Shadow Vehicle Moving Operation (b) | |||||
|---|---|---|---|---|---|
| Vehicle Weight (lb) | Prevailing Speed (mph) | Weight of Impacting Vehicle To Be Contained (a) | |||
| 4,500 lb | 10,000 lb | 15,000 lb | 24,000 lb | ||
| 10,000 | 60-65 | 100 Ft. | 175 Ft. (c) | 225 Ft. | 275 Ft. |
| 50-55 | 100 Ft. | 150 Ft. (c) | 175 Ft. | 200 Ft. | |
| 45 or less | 75 Ft. | 100 Ft. (c) | 125 Ft. | 150 Ft. | |
| 15,000 | 60-65 | 75 Ft. | 150 Ft. | 175 Ft. | 225 Ft. |
| 50-55 | 75 Ft. | 125 Ft. | 150 Ft. | 175 Ft. | |
| 45 or less | 50 Ft. | 100 Ft. | 100 Ft. | 100 Ft. | |
| 24,000 | 60-65 | 75 Ft. | 100 Ft. | 150 Ft. | 175 Ft. |
| 50-55 | 50 Ft. | 75 Ft. | 100 Ft. | 150 Ft. | |
| Notes: |
a) Weights of typical vehicles: mid-size auto, 2,250 lb; full-size auto 3,500 lb; loaded ¾-ton pickup truck, 6,000 lb; loaded 1-ton cargo truck, 10,000 lb; loaded 4-yard dump truck, 24,000lb. b) Distances are appropriate for the shadow vehicle speeds up to 15 mph. c) Values suggested as the appropriate buffer distance for vehicles equipped with TMAs. | ||||
In 1991, two TTAP instructor alumni, Jack Humphreys and Darcy Sullivan addressed the issue when they created the following information published in the NCHRP Report 337 "Guidelines for the Use of Truck-Mounted Attenuators in Work Zones." The document can be found at http://pubsindex.trb.org/view/1991/C/364851 and these guidelines are still current today. In the report, recommendations were offered for the assignment of Shadow Vehicles and for the application of TMAs.
Whenever TMA's are used, Roll-Ahead-Distance (RAD) must be included. Not all the impacting energy is absorbed by the TMA's crash cushion. The RAD is a safe following or parking distance, between the protected vehicles / workers / work space and the TMA, required to allow for roll-ahead of the TMA due to the impact from behind. Imagine the consequence of your workers being within that roll ahead space. As well, too great of length of RAD can mislead drivers to think they can re-enter the lane between the TMA and the workers. To counter this situation, TMA manufacturers include specific recommended RAD information within their operators' manuals. The following tables (provided in the report by Jack Humphreys and Darcy Sullivan) offer generalized acceptable RAD spacing. Keep in mind that actual RAD separation should consider the factors of TMA combined weight, TMA speed, as well as the maximum speed and design weight of the potential impacting vehicle.
As you can see, the RAD can be quite considerable. These recommended distances can be difficult for the TMA operator to maintain in actual mobile operations. Keep in mind that RAD conditions also apply when the TMA is parked, although the distances may be shorter.
Recent technology is allowing an option to this segment of Work Zone Safety. An Autonomous Truck Mounted Attenuator (ATMA) vehicle is now being tested in real world situations. A current article in "Equipment World's Better Roads" provides additional information. It mentions that the Autonomous Truck Mounted Attenuator (ATMA) combines driverless capability on a truck with a rear attenuator safety feature, designed to cushion impact in the event of a vehicle crash. In addition to remote driving, the new truck uses what's described as "leader-follower" technology, which allows it to follow another vehicle, mimicking the speed and turns of the lead vehicle. You can find the full article at: http://www.equipmentworld.com/florida-dot-to-test-autonomous-truck-mounted-attenuators-in-work-zones-later-this-year/.
The ATMA will be able to maintain RAD by the electronic/mechanical interface of the autonomous software and will remove the driver from the TMA vehicle and its inherent hazards. Therefore, the ATMA will be able to reduce worker hazard by adding another segment to protection. Will it be expensive? Yes, without doubt. However, if the ATMA concept is able to perform in field situations and conditions, this will allow the removal of human workers from possible injury.
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