Mr. Michael Conner, Vice President
110 West 22nd Street
P.O. Box 3010
Big Spring, Texas 79721
Dear Mr. Conner:
Thank you for your letter, received April 30, requesting Federal Highway Administration (FHWA) acceptance of your company's triangular slip base as a breakaway sign support system for use on the National Highway System (NHS). Accompanying your request was a letter report from the Texas Transportation Institute and videos of the crash tests. You requested that we find the slip base acceptable for use on the NHS under the provisions of National Cooperative Highway Research Program (NCHRP) Report 350 "Recommended Procedures for the Safety Performance Evaluation of Highway Features."
Testing of the supports was in compliance with the guidelines contained in the NCHRP Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. Requirements for breakaway supports are those in the American Association of State Highway and Transportation Officials' Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals.
Low speed pendulum testing was conducted on your company's devices. The mass of the test vehicle was 820 kg in all tests. The complete devices as tested are shown in the Enclosures. The pipe used as the sign support had an outside diameter of 2.875 inches, and a wall thickness of 0.120 inches, (the Texas Department of Transportation standard sign post pipe).
The Low Profile Mounting tested in UAS P4 was bolted directly to the "ground". The Concrete Foundation in UAS P5 was 8 inches in diameter and 30 inches deep, and was cast in NCHRP 350 "weak" soil. Slip-base bolts were torqued to 40 ft-lbs.
|Test #||NCHRP 350||Speed||Version||Occup. Speed||Delta V|
|UAS P4||3-60||33.6 km/h||Low Profile Mounting||1.1 m/s||0.59 m/s|
|UAS P5||3-60||35.2 km/h||Concrete Footer in S2 Soil||1.5 m/s||0.51 m/s|
|UAS P4||Extrapolation||100 km/h||Low Profile Mounting||0.50 m/s|
|UAS P5||Extrapolation||100 km/h||Concrete Footer in S2 Soil||0.47 m/s|
Occup. Speed: Occupant Impact Speed: Speed at which a theoretical front seat occupant will contact the windshield. In meters per second. Delta V: Speed change of the test vehicle. In meters per second.
Velocity changes were all within acceptable limits, and the only stubs remaining were the low profile base plate in test UAS P4, the top surface of which was 2 inches above the "ground", and the lower slip plate in test UAS P5, the top of which was 4 inches above the soil. Although interface washers were not used during the test you require that they be used in service. This is acceptable as the interface washers can only improve the breakaway performance and are standard hardware on most other slip base systems.
The results of testing met the FHWA requirements and, therefore, the devices described above and shown in the enclosed drawings for reference are acceptable for use as Test Level 3 devices on the NHS under the range of conditions tested, when proposed by a State.
Please note the following standard provisions that apply to FHWA letters of acceptance:
Michael S. Griffith
Acting Director, Office of Safety Design
Office of Safety