March 11, 2003


Mr. Albert W. Unrath Sr.
PO Box 317
Line Lexington, PA

Dear Mr. Unrath:

In your December 27, 2002 letter to Mr. Frederick G. Wright, Jr., former Director of the Office of Highway Safety, you requested the Federal Highway Administration's (FHWA) acceptance of a modified U-MAD truck mounted attenuator (TMA) at the National Cooperative Highway Research Program (NCHRP) Report 350 test level 1 (TL-1). The Federal Highway Administration has previously accepted both a TL-2 unit (U-MAD 70K) with a length of 7.5 feet and a TL-3 unit (U-MAD 100K) a total length of approximately 10 feet with for use on the National Highway System.

The modified TMA design, called the U-MAD 50K, consists of an aluminum box with internal compartments filled with variable density energy-dissipating material.  The first 24 inches is comprised of the softest material, followed by 21 inches of a stiffer material, and ending with 3 inches of a yet stiffer composition.  The unit is rectangular, measuring 1220-mm (48-in) long by 2286-mm (90 inches) wide.  It is 711-mm (28-in) deep and is 317 mm (12.5 in) above the roadway surface when deployed. 

To support your request, you included an analysis of the results of your earlier 70 km/h and 100 km/h tests and extrapolated these results to the shorter TL-1 design.  Since the impact energy to be dissipated in a 50 km/h test is approximately half that in a 70 km/h impact and the first two feet of all three designs consists of the same crushable material, followed by 18 or 21 inches of a stiffer material, I agree that the U-MAD 50K would likely meet the evaluation criteria for an NCHRP Report 350 TMA at the TL-1 speed of 50 km/h.  Therefore, it may be used on the National Highway System (NHS) when such use is deemed appropriate by the contracting authority when expected impact speeds are near or less than 50 km/h.

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

Sincerely yours,

(original signed by Michael S. Griffith)

Michael S. Griffith
Acting Director, Office of Safety Design
Office of Safety