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FHWA Highway Safety Programs


Any device that is used to display the traffic sign is known as the sign support, which consists of the vertical post and, if needed, any stiffeners onto which the sign panel is attached. Most posts for roadside post-mounted signs are one of the following types:

  • Wood post.
  • U-channel steel post.
  • Square or round tube steel post.
  • I-beam steel post.

A sign support can become a deadly hazard when struck by a vehicle that drives onto the roadside; therefore, there is an MUTCD standard that requires all roadside sign supports in the clear zone (see Appendix B for description of clear zone) to be breakaway, yielding, or shielded by a barrier or crash cushion. ‘Breakaway’ is a term to describe crash tested sign supports that break or bend upon impact. This includes sign supports that, when struck by a vehicle, separate from the base and are knocked ahead of or up and over the errant vehicle. A ‘yielding’ support refers to a support that bends, allowing a vehicle to run over it. Barriers are typically not installed to shield roadside signs, especially on local roads; hence, the design of the post, regardless of the type, must provide the breakaway or yielding feature. All sign supports on highways within the clear zone must either be of a breakaway type meeting the crashworthiness criteria of NCHRP 350 (see references) or be shielded by guardrail, barrier, or an energy absorbing system meeting NCHRP Report 350 or the AASHTO Manual for Assessing Hardware (MASH) criteria. Additional information on acceptable breakaway sign supports can be found at FHWA’s Office of Safety website:

Wood Post

Wood posts are frequently used, especially in regions where wood is economical compared to metal type supports. These posts usually come in sizes of 4 x 4 inches to 6 x 8 inches. All posts above 4 x 4 inch nominal size must be drilled perpendicular to traffic flow to allow the post to break away if struck by a motor vehicle.

Small supports have a cross section of approximately 16 square inches. This type of post should be buried about 30 to 36 inches deep depending upon the type of soil and backfill; check State specifications or with LTAP for embedment depth and backfill material. The post may need to be buried even deeper to reduce vandalism and reduce dislodging by heavy winds. One or two posts may be used to make up the sign support, but adding posts too close together can affect crashworthiness.

"Photo. A small wood sign support."

Small wood support is typically a 4 x 4 inch post.

Large supports should be drilled to provide the breakaway characteristic. For example, a 6 x 8 inch wood post can be used if the cross section is weakened by drilling two 3-inch holes (drill perpendicular to roadway). A 4 x 4 inch wood post is the largest undrilled wood post recommended to act as a breakaway support.

"A large wood sign post with two holes drilled in it at 4 and 14 inches above the ground."

Holes are needed for wood posts greater than 4 x 4 inches.

U-Channel Steel Post

The U-channel, hot rolled steel post is another common small sign support. It is considered breakaway since it will bend, break or pull out of the ground when it is hit.

Post Support. The post should be driven into the ground and not encased in concrete. A broken or damaged post is easier to remove if it is not driven or set into the ground more than 3.5 feet.

"Photo. A typical u-channel post."

Typical u-channel post.

Breakaway Devices. A U-channel post of re-rolled rail steel weighing 3 pounds-per-foot or less meets breakaway requirements by itself. If a heavier post is used, splices can be purchased commercially to install at ground level; alternatively a stub post of the same material can be set in a concrete base with a 4-inch length available to bolt to the sign post as a base connection. These devices improve safety when the post is hit, will make repair easier, and will make it possible to use a U-channel post when it has to be placed in a concrete area.

"Photo and diagram. The photo depicts a breakaway treatment for a u-channel post.""Photo and diagram. The diagram is of the same, showing the base below the ground, the post, and the 4-inch overlap between the two."

Breakaway treatments for u-channel posts.

Square Steel Tube

Another sign post is the square steel tube (perforated) design, which is used in many localities. Posts of this type are also considered breakaway if they are 2 ¼ inches or less in size.

Post Support. Posts can be driven into the ground. Do not place concrete around the post. A broken or damaged post is easier to remove if it is not driven or set into the ground more than three feet.

"Photo. A typical square steel post is shown."

Typical square steel post.

Breakaway Devices. As with the U-channel post, sleeve assemblies can be used for the base or slip couplings can be used near the base. These devices will increase the safety of a sign if it is hit and make it easier to repair. After the sign has been hit, the broken stub of the post can be removed from the base sleeve and a new sign post put back in place.

"Photo of example breakaway square steel posts is shown. This is a photo of a sleeve assembly.""Photo of example breakaway square steel posts is shown. This is a photo of a slip coupling."

Two types of breakaway designs.

I-Beam Steel Post

This post type is used when it is necessary to support large sign panels, which is common for roadside post-mounted guide signs.

All large steel posts use a breakaway feature, unless protected by barrier or placed out of the clear zone. This is usually accomplished by using a slip base that connects the post to the foundation. When struck, the post slips off the foundation at the bottom, and rotates around the hinge plate below the sign panel. This allows the vehicle to safely pass under the sign after impact.

"This photo depicts a large sign supported by I-beam steel posts.""This photos depicts a close up of the base of the posts."

I-beam steel posts with a slip base are needed for larger roadside guide signs.

There are many other products available commercially for sign supports. Use depends on local requirements and costs. This publication only shows four of the most common types of small sign supports. The comprehensive guide to breakaway sign supports is A Guide to Small Sign Support Hardware.