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December 21, 2004

Refer to: HSA-10/CC-64E

Mr. Albert W. Unrath Sr.
P.O. Box 317
Line Lexington, Pennsylvania 18932-0317

Dear Mr. Unrath, Sr:

In your November 29 letter, sent to Mr. Richard Powers' attention, you requested the Federal Highway Administration's (FHWA) acceptance of a modified design of your previously accepted 229-cm (90-inch) long test level 2 (TL-2) U-MAD 70K truck mounted attenuator (TMA). Specifically, you requested acceptance of a shorter unit (198-cm or 78-inches long) at the same test level.

To support your request, you stated that your earlier 70-km/h test with an 820-kg car (into the longer U-MAD 100K unit) resulted in a TMA penetration of approximately 150 cm (60 inches) and concluded that a 183-cm (72-inch) long unit with the same density in the crushed area would meet TL-2 evaluation criteria with the small car. You then ran the TL-2 pickup truck test 2-51 into a 183-cm (72-inch) long unit. The results of this test were contained in the Transportation Research Center's December 2003 - January 2004 report entitled "NCHRP Report 350 Test 2-51 of the U-MAD 70K Truck Mounted Attenuator". The truck impacted the TMA at 67.4 km/h. Occupant impact velocity was reported to be 9.4 m/sec and the ridedown acceleration was 16.5 g's. The 8595.6 kg support vehicle rolled ahead 3.9 meters. However, the test report noted that the "total crush of the TMA was not recorded due to the impacting vehicle's ...crushing the TMA and ultimately bending and breaking the cushion's lift plate".

Although meeting Report 350 evaluation criteria, testing revealed that a 183-cm (72-inch) long U-MAD 70K TMA had essentially no reserve capacity and its mounting hardware was damaged in the crash. Consequently, you requested the FHWA acceptance of a 198-cm (78-inch) long unit at TL-2. I agree that the 198-cm (78-inch) long U-MAD shown in the enclosure to this letter may be assumed to meet Report 350 evaluation criteria for a TL-2 TMA and may be used on the National Highway System (NHS) when such use is deemed appropriate by the contracting authority.

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


Sincerely yours,

(Original Signed By R. Powers)
John R. Baxter, P.E.
Director, Office of Safety Design
Office of Safety

2 Enclosures

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