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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter B-175

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U.S. Department of Transportation
Federal Highway Administration

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

June 25, 2008

In Reply Refer To: HSSD/B-175

Ronald K. Faller, Ph.D., P.E.
Research Assistant Professor
University of Nebraska Lincoln
527 Nebraska Hall
P.O. Box 880529
Lincoln, NE 68588-0529

Dear Dr. Faller:

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

You requested that we find this system 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.”

Requirements
Roadside safety systems should meet the guidelines contained in the NCHRP Report 350, "Recommended Procedures for the Safety Performance Evaluation of Highway Features". FHWA Memorandum “ACTION: Identifying Acceptable Highway Safety Features” of July 25, 1997 provides further guidance on crash testing requirements of longitudinal barriers.

Description
The MGS was first recognized in the FHWA Acceptance Letter B-133 dated March 1, 2005. In that letter it was described as follows:

The MGS barrier consists of standard 12-gauge W-beam sections installed with the top of the rail set at a nominal height of 787 mm (31 inches). It is mounted on standard W152 x 13.4 steel posts that are 1829-mm (6-feet) long and set on 1905-mm (75-inch) centers. To obtain the additional rail height, each post is raised approximately 100 cm (4 inches), resulting in slightly less embedment than the posts in a standard W-beam installation. The rail is offset from these posts with 152-mm wide x 305-mm deep x 356-mm long (6-inch x 12-inch x 14-inch) wood offset blocks. Finally, the rail splices are located at mid-span between adjacent posts rather than at each post as in a traditional W-beam installation.

Your present request is to allow the use of round wood posts of Douglas Fir (DF), Ponderosa Pine (PP), and Southern Yellow Pine (SYP) based on research conducted by the Forest Products Laboratory (FPL) and full-scale crash testing you conducted. The FPL research developed the following grading specifications for the three varieties of wood posts.

Species DF PP SYP
Maximum Knot Diameter 2 in 4 in 2.5 in
Minimum Ring Density 6 rings/in 6 rings/in 4 rings/in
Nominal Post Diameter* 7.25 in 8.00 in 7.50 in
Post Diameter Range* 7.00 to 8.00 in 7.75 to 8.75 in 7.25 to 8.25 in

*Post diameter at the ground line.

Crash Testing
The MGS barrier was subjected to two NCHRP Report 350 3-11 crash tests, MGSDF-1 using Douglas Fir posts, and MGSPP-1 using Ponderosa Pine posts. Because of prior testing of the MGS using SYP posts, tests on other w-beam barrier systems using round SYP posts, and the initial MGS test using the 820C vehicle (reported in B-133 noted above) only the two 3-11 tests were deemed necessary. Test Data Summary Sheets for the two tests are enclosed for reference. The occupant risk criteria were met for both impacts as was the post-impact trajectory, with no potential noted for spearing or deformation of the passenger compartment.

For the test on the Douglas Fir system (test no. MGSDF-1), the W-beam rail located upstream of the impact region had fallen to the ground after the test. For the test on the Ponderosa Pine system (test no. MGSPP-1), a smaller portion of the W-beam rail located upstream of the impact region had fallen to the ground after the test. The maximum dynamic deflection in test MGSDF-1 was 60 inches, and in test MGSPP-1 was 38 inches. For comparison, the maximum dynamic deflection in test NPG-4 using steel posts, reported in B-133, was 43 inches.

Findings
The MGS system described above using DF, PP, and SYP posts 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 the highway agency.

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

 

Sincerely yours,

Signature of David A. Nicol

David A. Nicol, P.E.
Director, Office of Safety Design
Office of Safety

Enclosures

Page last modified on June 24, 2011.
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