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FHWA Home / Safety / Roadway Departure / Sign Supports

Sign Supports

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Breakaway Supports - General

From this page you can reach FHWA Eligibility Letters dealing with breakaway sign supports, mailboxes, and delineators. A table summarizing the most common breakaway sign supports was included in NCHRP Web Only Report W-157 The criteria for breakaway supports focuses focused on the velocity change of the impacting vehicle and the height of the stub remaining after impact. Hardware that breaks away without slowing the vehicle by more than 5 meters per second (about 15.4 feet per second) and leaves a stub with no substantial remains taller than 100 mm (4 inches) may be eligible for reimbursement under the Federal Aid Highway Program. The AASHTO Manual for Assessing Safety Hardware has added the assessment of  windshield damage and roof crush in tests that use  both a small car and a pickup truck.

By using the pull-down menu above you can select the type of breakaway support you are interested in, or look at letters dealing with mailboxes or delineators.


Breakaway supports that are placed near intersections or other locations where errant vehicles may come at them from all directions must be of an omni-directional design. "Omni-directional" means that the support is symmetrical and will break safely when struck from any direction, or it has been specifically designed to function properly at all angles. Certain generic bases like the rectangular four-bolt slip base, or the inclined slip base are not designed to be omni-directional and will act like a non-breakaway support if struck from the side. Triangular three-bolt slip bases, wood supports, perforated square steel tube, spliced U-channel supports, and breakaway couplings, among others, are omni-directional.


Because most delineator posts are shorter and smaller gauge versions of crash-tested sign supports, no separate listing has been made. A few proprietary delineator supports of other materials have been crash tested and have received FHWA Eligibility Letters.


Like delineators, many mailbox supports are smaller versions of breakaway sign posts. The principal guidance for mailboxes is contained in the AASHTO Roadside Design Guide Chapter 11 "Erecting Mailboxes on Streets and Highways." This chapter includes details on generic crash tested supports that are acceptable for use. The Roadside Design Guide is available for purchase from the AASHTO Bookstore, which may be accessed from www.aashto.org

The FHWA is also preparing guidance on "secure" and "vandal-resistant" mailboxes, which are much heavier than the conventional rural mailboxes. Crash testing and computer simulation have shown that these heavy mailboxes must be firmly attached to a steel post that is solidly anchored to the ground. Otherwise the mailbox and/or the support can separate and penetrate the vehicle's occupant compartment.

Windshield damage

Windshield damage is not a pass/fail criterion for ground mounted breakaway supports like it is for portable work zone traffic control devices. Certain base-bending/yielding steel supports have caused substantial damage to the windshield of the test vehicle, but the sign and its support did not intrude a significant distance into the occupant compartment. The eligibility letters for these devices indicate their performance was "marginal, but acceptable."


The FHWA has mandated breakaway supports within the clear zone on NHS routes since 1998. In 2000 the MUTCD made breakaway sign supports a "shall" condition for supports within the clear zone of all streets and highways in the United States. FHWA intends to limit the mandate for breakaway supports on non-NHS routes to those posted at 50 mph or greater, and has set a deadline of January 2013 to accomplish the retrofit. Any retrofit of sign supports should be done in conjunction with the establishment of retroreflectivity requirements.

Page last modified on September 30, 2015.
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