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March 1, 2005

In Reply Refer To: HSA-10/CC-27A

Mr. Don Robertson
Marketing Regional Director
Engineered Arresting Systems Corporation
2550 Market Street
Aston, Pennsylvania 19014-3426

Dear Mr. Robertson:

In May 1995, a 36-inch diameter ESCO Water Twister Vehicle Arresting System (VAS) with a 60-foot wide net was accepted for use on the National Highway System as an NCHRP Report 350 attenuator at test level 3 (TL-3). This acceptance was based on successful completion of two full-scale crash tests conducted at the Texas Transportation Institute (TTI). Late last year, you requested acceptance of a smaller unit (18-inch diameter) with a 20-foot net based on an analytical comparison of the earlier design. Enclosures 1 and 2 show the key components of your Model 18 VAS and its connections to a 2.25-inch mesh by 42-inch high chain-link restraining net.

Dr. Dean Alberson, the research engineer at TTI who conducted the earlier full-scale tests of the 36-inch diameter VAS, reviewed your modeling procedure by comparing the actual and predicted occupant impact velocities (OIV's) and subsequent ridedown accelerations (G's) from the original tests with the predicted values from the Model 18 VAS unit. In his letter to Mr. Richard Powers dated February 22, he concluded that the OIV with the Model 18 VAS and the 1800-lb test vehicle would be 25.6 ft/s (7.8 m/s) and the ridedown G's would be 5.25. For the 4400-lb pickup truck, the corresponding values were estimated to be 19.5 ft/s (5.9 m/s) and 3.25 G's. All of these values are well within the bounds recommended in NCHRP Report 350 for a test level 3 (TL-3) attenuator. For both vehicles, the stopping distance was predicted to be approximately 50 feet.

Based on Dr. Alberson's review of your data and his conclusions noted above, the Model 18 VAS may be considered an NCHRP Report 350 attenuator at TL-3 and used on the National Highway System when such use is considered appropriate by the transportation authority. Since the Model 18 VAS has not been crash tested, initial installations should be evaluated in the field to verify acceptable crash performance. The rigid structures to which the VAS units are secured should be situated out of harm's way or adequately shielded from traffic.

Please note the following standard provisions that apply to 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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