December 20, 1996
Refer to: HNG-14/SS-68A
Mr. Lawrence E. Leahy
P.O. Box 135
Auburn, IL 62615
Dear Mr. Leahy:
This is in response to your October 24 letter to Mr. Seppo Sillan regarding your company's bolt-on triangular slip-base. Our September 18 Geometric and Roadside Safety Acceptance Letter SS-68 found a single support mounted in standard soil with a "winged anchor" soil plate acceptable for use on the National Highway System (NHS). You are now requesting that the Federal Highway Administration (FHWA) accept the base where three supports are used within a 2.1-meter width. You included a copy of an October 21 letter from the Texas Transportation Institute (TTI) that reported on a TTI analysis of the single-post crash tests and an extrapolation of the results to a three-post installation.
The TTI analysis indicates an occupant impact velocity of 2.7-m/s. Our own analysis indicates an occupant impact of 3.2 m/s, which meets, and is well below, the change-in-velocity requirement adopted by the FHWA. Therefore, your company's three-bolt slip-base for use in sign structures with one, two, or three 63.5-mm square perforated steel tube supports with soil plates is an acceptable breakaway device for use in standard soil on projects on the NHS if proposed by a State. All other conditions in our September 18 letter remain in effect.
Dwight A. Horne, Chief
Federal-Aid and Design Division
Geometric and Safety Design Acceptance Letter SS-68A