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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter SS-171

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U.S. Department of Transportation
Federal Highway Administration

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

May 2, 2011

In Reply Refer To: HSST/SS-171

Mr. Pratip K. Lahiri, P.E.
Manager, Specifications and Standards Section, POD 23
Design Quality Assurance Bureau
New York State Department of Transportation
50 Wolf Road
Albany, New York 12232

Dear Mr. Lahiri:

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: Aluminum Pedestrian Signal Pole
Type of system: Breakaway Support Structure
Test Level: NCHRP Report 350 TL-3
Testing conducted by: Midwest Roadside Safety facility
Date of request: November 30, 2010
Date completed package received: December 6, 2010
Request initially acknowledged: December 7, 2010

You requested that we find this system acceptable for use on the NHS under the provisions of the National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.”

Requirements
Roadside safety devices should meet the guidelines contained in the NCHRP Report 350 or the American Association of State Highway and Transportation Officials’ Manual for Assessing Safety Hardware (MASH). Requirements for breakaway supports are those in the American Association of State Highway and Transportation Officials’ (AASHTO) Standard Specifications for Structural Supports for Highway Signs, Luminaries, and Traffic Signals.

Decision:
The following device was found acceptable:
• New York State DOT Aluminum Pedestrian Signal Pole

Description
The aluminum pedestrian signal pole includes an aluminum pole and a pedestrian “hand/man” signal. The aluminum pole is attached to a rigid foundation by a frangible aluminum base plate and four bolts.

The pole is a 10 ft (3.1 m) tall, round aluminum pole with a 1/8 inch (3 mm) wall thickness. The pole has a top outside diameter of 4½ inches (114 mm) and a bottom outside diameter of 6 inches (152 mm). A 5/32 inch (4 mm) thick, 24 inches (610 mm) tall internal reinforcing sleeve is located at the bottom of the pole and serves to strengthen the base of the pole against premature yielding during an impact. A handhole is placed through both the pole and the internal sleeve and centered at a height of 18 inch (457 mm).

The pole base plate is a 10¼ inch (260 mm) square with a thickness of 5/8 inch (16 mm). The bolt circle is 9½ inches (241 mm) in diameter, and the pole was inserted and welded to a 3½ inch (89 mm) tall cylinder, as measured from the bottom of the base plate. Enclosure 1 and Enclosure 2 illustrate the details of the pole.

The pedestrian “hand/man” signal conformed to Standard Sheet No. 680-10 used by New York State DOT. The signal is mounted to the pole using a top-mounted attachment bracket, as shown in Enclosure 3. The signal is attached to the bracket by inserting the top, threaded portion of the bracket into the hole at the bottom of the signal. The combined weight of the signal and bracket was 26 lb (12 kg). After attachment to the pole, the total system weighed 59 lb (27 kg).

Crash Testing
A pendulum testing was conducted on the test articles described above by Midwest Roadside Safety facility. The pendulum test was conducted according to NCHRP 350 test designation 3-60 as a surrogate for a full crash test with an 820C car.

Findings
According to NCHRP 350, test designation 3-60 and test designation 3-61 are to be conducted for support structures for Test Level 3 acceptance. In both tests full scale automobile testing with 820C small car is required.

In this request, test 3-60 was conducted using a pendulum. The mass of the pendulum was 861 kg and the pendulum was configured to represent the front-end crush stiffness of a 1979 Volkswagen Rabbit two-door sedan as a small car.

According to the test results, as reported by the Midwest Roadside Safety facility, the test article passed test 3-60 conducted using the pendulum. In this test, maximum vehicle velocity change was below the maximum allowable limits of NCHRP 350 (Enclosure 5). In this test, the pole broke away from the base plate assembly in a controlled and predictable manner. The signal pole fell in front of the surrogate vehicle, neither the signal box nor the fractured pole showed any potential for penetrating or causing larger deformations to the occupant compartment.

The test articles were not crash tested according to test 3-61 (high speed test). However, the results of the high speed tests were estimated using the results from the low-speed test in combination with an analytical extrapolation method described in FHWA Memorandum HNG-14 for Action: Identifying Acceptable Highway Safety Features dated on July 25, 1997. For the test article described above, the high speed test extrapolation analysis resulted in change in velocity values for the high speed test below the NCHRP 350 limit of 5.0 m/s. Therefore, the test article would likely successfully pass the high speed test.

The AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries, and Traffic Signals require that any “substantial remains” of the breakaway support not project greater than 4 inches above ground level to avoid vehicle undercarriage snagging. After the test portions of the fractured aluminum base plate assembly projected 4½ inches (114 mm) which violates the 4 inch (100 mm) stub height limit. Therefore, the test article does not pass all requirements set forth by FHWA.

In your request letter, you proposed two alternative modifications to the system design in order to overcome the issue associated with the remaining stubs:

  1. The leveling nuts, which are 1 inch high, will be specified to be embedded in the grout as shown in Enclosure 4 (Option 1). This will ensure that the distance from the ground to the top of the collar, assuming the same break point, will be less than 4 inches high.
  2. The concrete foundation used for the system will be specified to be level with the surrounding ground and any sidewalk. In addition, in order to ensure that the maximum height to the top of the collar from the foundation does not exceed 4 inches, two alternatives can be implemented:
    1. The existing collar height is kept (3 ½ inches) and shims (Enclosure 5) are used to level the pole instead of nuts; or
    2. The height of the collar is reduced to 3 inches and 1 inch or 7/8 inch high leveling nuts are used.

I concur in either of the proposed modifications on the grounds that they can ensure that the remaining stubs will not exceed the 4 inch limit.

Therefore, 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 George E. Rice, Jr. */

for

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

Enclosures

Page last modified on December 17, 2013.
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