Federal Highway Administration
400 Seventh St., S.W.
Washington, D.C. 20590
July 12, 2007
In Reply Refer To: HSSD-10/B-161
Mr. Pratip Lahiri, P.E.
Specifications and Standards Section, POD 23
New York State Department of Transportation
50 Wolf Road
Albany, NY 12232
Dear Mr. Lahiri:
Thank you for your letter requesting the Federal Highway Administration’s (FHWA) acceptance of a four strand cable median barrier as a Test Level 3 device for use on the National Highway System (NHS). The median barrier is a modification of your previously accepted generic three strand cable system. You requested that we find this barrier 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.”
The FHWA guidance on crash testing of roadside safety hardware is contained in a memorandum dated July 25, 1997, titled “INFORMATION: Identifying Acceptable Highway Safety Features.”
Generic low-tension 3-cable barriers have been tested with various cable heights and spacing. The New York State Department of Transportation cable median design has the top cable at 28 inches above grade with the cables spaced 6 inches apart. The Federal Outdoor Impact Laboratory recently conducted physical testing and computer simulations of cable median barrier designs. These indicated a need for the bottom cable to be significantly lower than most designs currently in use. Your modification is in conformance with the recommendations of that research. Those recommendations indicated a need for the cable median barrier to have four cables, spaced 6 inches apart vertically, and mounted on alternating sides of the post with the lowest cable on the downhill side.
You acknowledged various limitations on the use of cable barrier systems. Once struck, the entire run is effectively compromised until the damage is repaired and the system retensioned. Winter repairs are often deferred until spring due to the inability to drive replacement posts in the frozen ground.
Cable tension under varying temperature conditions, and spacing between anchorages are detailed on the drawings.
Old installations used a 1 cubic yard anchor block that sometimes moved after several impacts. You doubled the block size and added a check of the anchor block orientation to your impact repair procedure to ensure that anchor blocks do not rotate enough that a cable end is released from its anchor angle slot.
Your request to add a fourth cable to your median barrier system includes a separate anchor connection for the added cable. This connection is identical to the connections for the other three cables and can be expected to perform in a similar manner. Because the addition of a fourth cable will serve to improve the performance of your median barrier system, with little potential for any adverse consequences, the system described above and detailed in the enclosed drawings is acceptable for use on the NHS as a test level 3 device.
Please note the following standard provisions that apply to the FHWA letters of acceptance:
George E. Rice, Jr.
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