November 13, 2002

Refer to HSA-10/SS-117
Mr. Stuart Cole
Northwest Pipe Company
Traffic Systems PO Box 2002

Houston, TX 77252-2002

Dear Mr. Cole:

Thank you for your letters of December 27, 2002, January 15, 2003, and February 10, 2003, requesting Federal Highway Administration (FHWA) acceptance of variations to your company's breakaway support systems for use on the National Highway System (NHS). Accompanying your letters were: 1) a report from the Texas Transportation Institute (TTI) on the square slipbase system, and videos of the crash tests; 2) a 1996 TTI report on weak soil testing of the PozLoc system; and 3) drawings of the PozLoc Breakaway Mailbox Support. You requested that we find the following 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."

1) Northwest Pipe POZ LOC S-Q Post TM Sign Support System.

2) PozLoc Sign Support System with extended anchor for use in weak soils.

3) PozLoc Breakaway Mailbox Support System for snow regions.


Testing of the Square Slipbase Sign Support System and prior testing of the PozLoc systems 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' (AASHTO) Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals.

1) The Northwest Pipe POZ LOC S-Q Post TM Slipbase Sign Support System consists of a 2.5 x 2.5 inch x 10 ga and 12 ga perforated square steel tube signpost mounted in a triangular slip base system. This system uses a 0.02 inch thick triangular keeper plate separated from the base plate by circular cardboard washers. A 3 ¾ inch long horizontal 0.5 inch diameter pin through the holes in the signpost prevent the end of the post from dropping below the slip joint.

In test NWP-P1, the lower portion of the slip base was rigidly anchored to the steel anchor plate in the pendulum facility. In test NWP-P2, the lower portion of the slip base was mounted in a standard TxDOT concrete footing placed in NCHRP Report 350 standard soil at the pendulum

Facility. Details of the triangular slip base system are detailed in the diagrams that are enclosed for reference. A 48 x 48 inch x 5/8-inch thick plywood sign panel was mounted at a height of 7 feet in test P1, and at a height of 7 feet, 2 inches in test P2. The slip base bolts were torqued to 40 foot-pounds in test P1 and to 60 foot-pounds in test P2


Pendulum testing was conducted on the slip base system. The mass of the test bogie was 820 kg in all tests. The test bogie was equipped with a crushable honeycomb nose simulating the front end of a Volkswagen Rabbit. (Although no longer produced, this configuration is considered a "worst case" design.) The complete devices as tested are shown in the Enclosures.


Test Number NWP-P1

Test Number NWP-P2

Foundation Type

Rigid test foundation

Concrete footer in Std soil

Pendulum Test Mass


Slip Base Bolt Torque

40 foot pounds

60 foot pounds

Impact Angle

Zero degrees

Zero degrees

Test Impact Velocity

21.1 mph (34 kmh)

21.2 mph (34.2 kmh)

Occupant Impact Speed



Bogie Delta V

1.32 fps (0.43 m/s)

1.47 fps (0.47 m/s)

Extrapolated high speed Delta V

1.84 fps (0.56 m/s)

1.73 fps (0.53 m/s)

Stub Height

4 inches

3-1/2 inches

Occup. Speed: Occupant Impact Speed: Speed at which a theoretical front seat occupant will contact the windshield. Delta V: Speed change of the test vehicle / bogie.


Velocity changes were all within acceptable limits.Because stub heights were at the 4-inch maximum we recommend that installation instructions stress the need to keep the height of the lower slip plate below the 4 inch maximum as required by the AASHTO Specification. The results of testing met the FHWA requirements and, therefore, the Northwest Pipe Square Slipbase Sign Support System 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.

2) PozLoc Sign Support System with extended anchor for use in weak soils. The PozLoc sign support system was crash tested in weak soils in February of 1997. In both the low and high speed tests the post pulled the socket / anchor out of the ground (in strong soil, the post pulls out of the socket.) FHWA Acceptance Letter SS-65A, dated 6-20-97, covered the PozLoc system in all soil types. The PozLoc anchor used in the testing was 33 inches (383 mm) long. Your current request is to find this system acceptable if anchors 48 inches (1220 mm) or 60 inches (1525 mm) long are used. Because of the increased embedment length it can be assumed that there will be greater resistance to pullout. After reviewing the prior crash testing we have concluded that the greater resistance will not be a disadvantage. If the anchor does not pull out then the system's

performance will be more like that seen in the strong soil testing. In those tests, the subject of FHWA Acceptance Letter SS-1 dated 5-13-86, the vehicle velocity changes were also below the " desirable" limits. Therefore, we concur that 48-inch and 60-inch long PozLoc anchors are acceptable for use.

3) PozLoc Breakaway Mailbox Support System for snow regions. The Northwest Pipe Co. 90 Degree Poz-Loc Socket Assembly for use with mailboxes is similar to the POZ-LOC Socket System with a minor difference. The single box cantilevered support system currently being used is 1-1/4 inch Schedule 40 pipe fabricated with pipe fittings (drawings enclosed for reference) in a concrete base. You proposed that the post be a 2-3/8 inch OD 14 ga pipe using a swaged elbow. The post is to be mounted in a POZ-LOC Socket and wedge mounted in concrete. You also requested a different single and double box support, which is a simple vertical 2-3/8 inch OD

14 ga pipe, mounted on a POZ-LOC post and socket. Because the Poz-Loc anchor performance has been found acceptable for sign supports, and the thin 14 gage steel is specified for the pipe supporting the mailbox, these systems can be expected to perform in an acceptable manner.


In addition to the square slipbase system discussed as item 1) above, the Poz-Loc supports covered as items 2) and 3), as 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, or under the range that similar systems were tested, when proposed by a state.

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

Sincerely yours,

Michael S. Griffith
Acting Director, Office of Safety Design
Office of Safety

2 Enclosures