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U.S. Department of Transportation

Federal Highway Administration

400 Seventh St., S.W.
Washington, D.C. 20590

April 15, 2005

In Reply Refer To: HSA-10/CC-90

Dean L. Sicking, P.E., Ph.D.
Safety By Design Company
6307 Yellowstone Circle
Lincoln, Nebraska 68510

Dear Dr. Sicking:

In your March 10 letter, you provided design and crash test information on a Trailer Truck-Mounted Attenuator (TMA) and requested formal acceptance of this device for use on the National Highway System (NHS) at the National Cooperative Highway Research Program (NCHRP) Report 350 test level 3 (TL-3). To support your request, you also sent me a copy of the Midwest Roadside Safety Facility's February 28 report entitled “Performance Evaluation of the Trailer-Truck-Mounted Attenuator (Trailer TMA) – NCHRP 350 test nos. 3-50 and 3-51 (TTMA-3 and TTMA-4)” and videotapes of the crash tests conducted.

The Trailer TMA design is based on the bursting tube technology originally developed for the energy-absorbing terminal for box-beam guardrail and subsequently adopted for use on trailer-mounted changeable message signs/arrow boards. The Trailer TMA includes four major components: an impact head assembly, a trailer frame, a hitch assembly, and a breakaway axle assembly. The trailer frame incorporates three levels of energy absorption: (1) a 610-mm (24-in) long bursting tube fabricated from 152 mm x 152 mm x 3.2 mm (6 in x 6 in x 1/8 in) ASTM A500 Grade B structural tube, (2) a 2594-mm (9-ft, 6.125-in) long second stage bursting tube of the same size and material, and (3) an 1834-mm (6-ft, 3/16-in) 152 mm x 152 mm x 4.8 mm (6 in x 6 in x 3/16 in) ASTM A500 Grade B structural tube. The general design of the Trailer TMA is shown in Enclosure 1. As noted therein, the total length of the tested design is 7050 mm (23 ft, 1-9/16 in) and its width is 2438 mm (8 ft) at the impact plate assembly. The trailer frame is 464 mm (18.25 in) above the ground and the total weight of the TMA is approximately 640 kg (1410 lbs). I am assuming that users may obtain detailed drawings directly from you and that such drawings will accurately depict the device that was crash tested.

The safety performance of the Trailer TMA was verified through full-scale crash testing. Both mandatory NCHRP Report 350 tests required for a truck-mounted attenuator were conducted: the 820-kg car (test 3-50) and the 2000-kg pickup truck (test 3-51) impacting head-on at 100 km/h. Summary sheets for these two tests are shown in Enclosure 2. In both tests, the support truck was blocked to prevent any forward movement. Thus, there is no upper limit to the weight of the support vehicle with which the Trailer TMA is used.

Although TMAs may also be used with lighter support vehicles, potential users must be made aware that its use with a significantly lighter vehicle, while likely to decrease impact forces on the occupants of the errant vehicle, will increase the forces transmitted to the driver of a lighter support vehicle. It then becomes critical that the support vehicle has proper headrest supports and that the driver be securely belted. Furthermore, the support vehicle roll-ahead and the post-crash trajectories (particularly following off-center impacts) of both vehicles will differ considerably from those seen in tests with the “standard” 9000 kg (19,840 lbs) support vehicle. Thus, it is not recommended that any TMA be used with a support vehicle as light as a pickup truck without test verification.

You have also submitted results of a bogie test showing an offset impact with the device similar to optional test no. 3-52. While the results of the bogie test indicate that the device may meet Report 350 evaluation criteria under offset impact conditions, the two optional tests (nos. 3-52 and 3-53) recommended in Report 350 should be conducted to confirm that the device will perform satisfactorily in these two tests.

Based on the above, I conclude that your tested design meets the NCHRP Report 350 evaluation criteria for tests 3-50 and 3-51 and may be used on the NHS if such use is acceptable to the contracting agency.

Please note the following standard provisions that apply to the FHWA letters of acceptance:

Sincerely yours,

/original signed by/

John R. Baxter, P.E.
Director, Office of Safety Design
Office of Safety

2 Enclosures

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