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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter SS-166

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U.S. Department of Transportation
Federal Highway Administration

1200 New Jersey Ave. S.E.
Washington, D.C. 20590

December 7, 2010

In Reply Refer To: HSSI/SS-166

Mr. Eric Hemphill, P.E.
Assistant Director of Maintenance Operations
North Texas Tollway Authority
P.O. Box 260729
Plano, TX 75026

Dear Mr. Hemphill:

This letter is in response to your request for the Federal Highway Administration (FHWA) acceptance of a roadside safety system for use on the National Highway System (NHS).

Name of system: North Texas Tollway Authority (NTTA) Short Sign Support
Type of system: Sign Supports — Reduced Height
Test Level: Manual for Assessing Safety Hardware (MASH) Test Level 3 (TL-3)
Testing conducted by: Texas Transportation Institute (TTI)
Task Force 13 Designator: SSP10a-b
Date of request: July 22, 2010
Date of completed package: November 23, 2010
Initially acknowledged: July 29, 2010

You requested that we find this system acceptable for use on the NHS under the provisions of the American Association of State Highway and Transportation Officials’ (AASHTO) Manual for Assessing Safety Hardware (MASH).

Decision:
The following device was found acceptable. Details are below:

Requirements
Roadside safety devices should meet the guidelines contained in the AASHTO MASH. The FHWA memorandum, “Identifying Acceptable Highway Safety Features” of July 25, 1997, provides further guidance on crash testing requirements of longitudinal barriers.

Description
The NTTA Short Sign Support is a post and sign system supporting a 2-ft high by 3-ft wide WRONG WAY sign mounted 2 ft above ground. The sign panel is 0.11 inch thick aluminum and is mounted on a single 60-inch long post consisting of a 2 3/8-inch diameter pipe support with a wall thickness of 13 gauge (0.092 inch). The pipe support is embedded in a 12 inch diameter by 30 inch deep concrete footing using a Poz-Loc wedge and socket device. Details of the system are shown in Enclosures 1 and 2. The sign is intended for use on main lane freeway exit ramps to alert drivers of wrong way movements.

Crash Testing
Test 3-61 was conducted on the sign support by TTI according to the MASH guidelines. The 1100C vehicle impacted the test article at 64.3 mph at 0 degrees. The testing was conducted with the test vehicle hitting the back of the sign post, as this orientation is likely to be the most common type of impact experienced in service, given that the majority of vehicular traffic towards the sign will be travelling the correct direction, leaving the highway. The crash test was evaluated in accordance with the criteria presented in the MASH. The test article successfully passed test 3-61. The NTTA short sign support yielded to the 1100C vehicle. In testing the sign panel separated from the post, vaulted, but did not penetrate the occupant compartment. The longitudinal and lateral Occupant Impact Velocities were 2.6 ft/sec and 0.3 ft/sec respectively which were below the MASH preferred value of 10 ft/sec. The maximum longitudinal and lateral Occupant Ridedown Accelerations were -0.3G and -0.2G respectively. Both values were well below the MASH preferred value of 15.0G. Enclosure 3 shows the results of the test results.

Findings
According to the MASH, test designations 3-60, 3-61, and 3-62 are to be conducted for approval of TL-3 for support structures. Test 3-60 which is a low speed impact test with a passenger car and test 3-62 which is a high speed impact test with a pickup truck were not performed. In your letter you requested that tests 3-60 and 3-62 be waived. You also requested that the TL-2 acceptance be granted. We concur with these requests as explained below.

Test 3-62 can be waived on the grounds that (1) the post will likely yield in the same manner that was observed in test 3-61 with a passenger car; (2) given the height of the hood of most pickup trucks, it will be unlikely that the sign panel can hit the windshield of a pickup truck; and (3) the Occupant Impact Velocities and Occupant Ridedown Accelerations were well below the preferred limits and those are not expected to be much larger for a pickup truck.

Low- speed tests including Test 3-60 and tests required for TL-2 approval are waived based on the performance of the test article observed in the video of the high speed test and the fact that the low speed tests have generally been to verify that the breakaway features perform as intended. In addition, the wedge and socket system has been tested an accepted at both low and high speeds as a full-height sign support.

In test 3-61, the sign panel separated from the post once the test vehicle impacted the sign. Therefore, use of the sign system should give consideration to proximate pedestrian activity. In the context of the comments provided, the system described in the requests above and detailed in the enclosed drawings is acceptable for use on the NHS under the range of conditions tested, when such use is acceptable to a highway agency.

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

 

Sincerely yours,

/* Signature of Michael S. Griffith */

Michael S. Griffith
Director, Office of Safety Technologies
Office of Safety

3 Enclosures

Page last modified on December 17, 2013.
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