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U.S. Department of Transportation

Federal Highway Administration

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

June 15, 2005

In Reply Refer To: HSA-10/CC-78C

Mr. Barry D. Stephens, P.E.
Sr. Vice President Engineering
Energy Absorption Systems, Inc.
3617 Cincinnati Avenue
Rocklin, California 95678

Dear Mr. Stephens:

Mr. Douglas Bernard recently delivered your June 7, 2005 letter to Mr. Richard Powers of my staff. In this letter, you requested the Federal Highway Administration (FHWA) acceptance of a modified version of your existing Safe Stop 180 TMA, specifically a new trailer version with wheels and a towing hitch. You refer to this unit as the Safe Stop Trailer TMA. To support your request you submitted drawings, an April 2005 test report prepared by E-TECH Testing Services, Inc. entitled “NCHRP Report 350 Crash Test Results for the Safe-Stop Trailer TMA” and crash test videotapes.

You stated that the Safe Stop Trailer TMA uses the same collapsible arms, impact frame, support frame and energy absorbing cartridges as the original Safe Stop 180 TMA previously submitted to our office for acceptance (reference FHWA letter HSA-10/CC78 & CC78A). The total length of this TMA is 5.77 m (18'-11”) including the trailer hitch, and its maximum width at the impact face is 2.36 m (7'-9”). It weighs 1186 kg (2615 lbs). Two modifications were made to the Safe Stop 180 to create the new Safe-Stop Trailer TMA: (1) the upward folding mid-frame elements and hydraulic system were replaced with a rigid frame that incorporates a suspended axle and wheels, and (2) a trailer hitch was added to the front of the unit along with a damper system that allows the system to articulate like a normal trailer behind the support vehicle but locks to prevent the trailer from rotating during off-set or angled impacts. These before-and-after modifications are summarized in Enclosure 1, where it is also noted that the two truck cylinder mounts and an optional truck pintle hook assembly are included with the TMA. The cylinder mounts must be aligned with the frame rails of the support vehicle and its bumper must be able to resist the anticipated test level 3 (TL-3) design impact loads to ensure optimal attenuator crash performance.

We agree these modifications and attachment details are unlikely to affect the test results previously submitted for the Safe Stop 180 TMA for the two required TMA tests, i.e., tests 3-50 and 51. At our request, you conducted optional test 3-52 to validate acceptable performance in the off-center impact that we believed to be the most demanding of the two optional tests currently recommended in the NCHRP Report 350. You subsequently decided to run optional test 3-53 as well to demonstrate that the trailer hitch dampening system would prevent the TMA from rotating even in an offset, angled impact into the rear of the unit. In both of these tests, the Safe Stop Trailer TMA did resist rotation and collapsed in a manner similar to that seen in your earlier end-on tests into the Safe Stop 180. The summary results of both supplementary tests with the Safe Stop Trailer TMA are shown in Enclosure 2.

Based upon our review of the information you sent us, the FHWA acknowledges that the Safe Stop Trailer TMA can be assumed to meet all evaluation criteria in the NCHRP Report 350 for tests 3-50 and 3-51, and specifically met these criteria in optional tests 3-52 and 3-53. It may be used as a TL-3 TMA on the National Highway System (NHS) when such use is acceptable to the contracting authority.

Please note also that the following provisions 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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