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Road Management & Engineering Journal
Copyright © 1997 by TranSafety, Inc.
April 1, 1997
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402
transafety@live.com

Making Intersections Safer for Pedestrians
Connecticut Truck-Mounted Attenuator Passed All Safety Requirements
Roadway Collapse Caused by Broken Water Main Was a "Special Defect": Texas City May Be Liable for Motorists' Injuries
Researchers Study the Walking Speeds of Older Pedestrians


















Connecticut Truck-Mounted Attenuator Passed All Safety Requirements

The Connecticut Truck Mounted Attenuator (CTMA) has passed four full-scale crash tests to meet all the performance requirements of NCHRP (National Cooperative Highway Research Program) Report 350. Authors John F. Carney III of Vanderbilt University in Nashville, Tennessee, Charles E. Dougan of the Connecticut Department of Transportation (ConnDOT), and Eric C. Lohrey of ConnDOT wrote a description of the crash tests and their results for a paper presented at the January 1996 meeting of the Transportation Research Board (TRB). They called the paper "Summary of the NCHRP Report 350 Crash Test Results for the Connecticut Truck-Mounted Attenuator (CTMA)." This article summarizes their report.

THE CTMA

ConnDOT began research to design and build a new CTMA in 1975. The authors described the motivation for this effort as concern for highway maintenance personnel who "were exposed to errant motorists during the course of normal workdays . . ." They further commented, ". . . [I]t was clear that an effective TMA could offer positive protection to maintenance and construction personnel performing field duties."

Early research at the University of Connecticut resulted in a design that used a cluster of mild steel cylinders to dissipate kinetic energy when a vehicle hit the CTMA. The four thin-walled steel cylinders deform on impact and dissipate energy effectively when struck by both lightweight and heavy vehicles.

The CTMA has three major components: (1) the service-vehicle guidance frame, (2) energy absorbing cylinders, and (3) the impacting plate assembly. The service- vehicle guidance frame attaches to the rear and sides of the highway maintenance vehicle and to the last steel cylinder. The cylinders themselves are made either of sections of seamless steel pipe or specially formed steel plate. The impacting plate mounts on the back of the CTMA assembly and is the surface a vehicle will hit in a collision with the back of a maintenance vehicle protected by the device. The plate is aluminum, and aluminum tubing on the sides of the plate slides inside the steel structural tubing to guide the collapsing frame during impact. This design functions satisfactorily during head-on and off-center impacts.

EARLY CRASH TESTS

From 1975 to 1978, six full-scale crash tests at Calspan Corporation and Texas Transportation Institute (TTI) examined the effectiveness of the CTMA. Since these tests predated NCHRP Reports 230 and 350, the tests were designed to comply with specifications in NCHRP Report 153 and Transportation Research Circular 191. At the time, there was no federally recommended TMA crash-test matrix.

Following existing guidelines, researchers conducted the six early crash tests using both heavy and light automobiles traveling just over 47 miles per hour (76 kilometers/hour) and hitting a CTMA mounted on a stationary maintenance vehicle which was in gear. Crash tests included head-on impacts with light and heavy vehicles, an offset impact with the heavy vehicle, and an offset impact at a ten-degree angle with the heavy vehicle.

The authors reported that test results revealed the CTMA's effectiveness in four areas:

  1. The CTMA dissipates the impacting energy in such a way that the accelerations to which the automobile and service vehicle are subjected are acceptably safe for the occupants of both vehicles.
  2. The unit is inexpensive to repair after an impact, requiring only that new cylinders be inserted into the system.
  3. There is no tendency for the impacting automobile to nose-dive under or catapult over the CTMA. In the event of an eccentric hit, intrusion of the impacting vehicle into adjacent traffic lanes is minimal.
  4. The CTMA is easily attached to and removed from the carrying vehicle. It is compact and designed for use in curved and hilly roads.
In 1990, the Texas State Department of Highways and Public Transportation asked TTI to develop performance criteria specifications for TMAs and recommend existing TMAs to adopt for use on Texas highways. TTI crash tested seven TMAs and recommended three for use in Texas. The CTMA was one recommended design.

RECENT CRASH TESTS

In 1993, national crash-test conditions for TMAs became available with publication of NCHRP Report 350 entitled "Recommended Procedures for the Safety Performance Evaluation of Highway Features." NCHRP Report 350 prescribed required and optional crash tests at speeds of about 43 mph (70 km/h) and 62 mph (100 km/h) and with 1,800-pound (820-kilogram) automobiles and 4,400-pound (2,000 kg) pickup trucks. The angle and offset of the impacting vehicle varied in the different tests. The report specified evaluation criteria for two aspects of the crash test results: the impacting vehicle and its occupants and the support vehicle and its driver. For both vehicles, major concerns are occupant risk and vehicle trajectory.

TTI did four crash tests (two required and two optional) with the CTMA. In each test, the CTMA "satisfied all of the requirements of NCHRP Report 350." Details of the four tests are given below:

Test No. 1, NCHRP Report 350 Test 2-50: Here an 1,800-pound automobile hit the CTMA head-on, with no offset, at 45 mph. Since the maintenance vehicle could not move forward during the test, it was placed with the front against a rigid barrier. Although TTI did this test in 1990 using guidelines from NCHRP Report 230, the procedures and evaluation criteria were "virtually identical" to those in NCHRP Report 350. The CTMA met all test requirements.

Test No. 2, NCHRP Report 350 Test 2-51: Here a 4,400-pound pickup hit the CTMA head-on with no offset. The maintenance vehicle could move forward during this test, and the impact-vehicle speed was about 44 mph. The CTMA handled the pickup's higher center of gravity, and the test results did not require making design changes.

Test No. 3, NCHRP Report 350 Test 2-52: In this test, a 4,400-pound pickup hit the CTMA with an offset of one-third of its width. The pickup's speed was about 44 mph. Again, the CTMA met or exceeded the requirements of all evaluation criteria.

Test No. 4, NCHRP Report 350 Test 2-53 This demanding, optional crash test required the CTMA to successfully attenuate the impact of a 4,400-pound pickup truck moving at about 43 mph. The impact was at a ten-degree angle with an offset of one- fourth of the vehicle width. Test results showed that occupant impact velocity and ridedown deceleration values were below preferred and required maximums cited in NCHRP Report 350.

SUMMARY

In four crash tests, the CTMA passed all NCHRP Report 350 requirements. No changes were needed in the original design. Given the successful test results, the Federal Highway Administration has approved the CTMA for use on federal-aid highway projects and on the National Highway System.

ConnDOT is currently using about 80 CTMAs. During more than 15 years of use on Connecticut highways, the CTMA has experienced "many severe impacts . . . involving a wide variety of vehicles, including a tractor-trailer rig." The authors report, "No serious injuries or deaths have occurred as a result of impacts with the CTMA. In the vast majority of cases, no injuries have resulted from CTMA impacts." One crash involved a jack-knifed tractor-trailer rig; no injuries resulted.

The CTMA is not a proprietary device; therefore, highway agencies may make and use the system without restriction. In addition, a Connecticut manufacturer is producing and selling the unit. For detailed crash-test information, video of tests performed, or complete design and construction details on the CTMA, contact:

Connecticut Department of Transportation
Office of Research and Materials
280 West Street
Rocky Hill, Connecticut 06067

Copyright © 1997 by TranSafety, Inc.


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