Minnesota Department of Transportation

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Crashworthiness - FAQs

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Crashworthiness - FAQs

Q: Why should I care about crashworthy sign supports and mailboxes?


A: As the road authority it is your duty to maintain streets in a reasonably safe condition. Rigid and fixed objects near the roadway can be a hazard and pose a serious danger in a crash. Crashworthy mailboxes and sign posts help to reduce these risks.

Q: How do I know if my sign posts are crashworthy?


A: Small signs (Type C and D) are typically supported by one of the following types: U-Channel Steel Post, Square Steel Tube or Wood Post. Some general information about each of these supports is provided below. Additional information is available at FHWA: Maintenance of Sign and Sign Supports - A Guide for Local Highway and Street Maintenance Personnel.

  • U-channel Steel Post: 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.
  • Square Steel Tube: As with the U-channel post, sleeve assemblies can be used for the base or slip couplings can be used near the base. Posts of this type are also considered breakaway if they are 2 ΒΌ inches or less in size.
  • Wood Post: These posts usually come in sizes of 4 x 4 inches to 6 x 8 inches. A 4 x 4 inch wood post is the largest undrilled wood post recommended to act as a breakaway support and a 6 x 8 inch wood post can be used if the cross section is weakened by drilling two 3-inch holes.

Q: What is the Clear Zone?

A: The AASHTO Roadside Design Guide defines a clear zone as the "total roadside boarder area, starting at the edge of the traveled way, available for safe use by errant vehicles. This area may consist of a shoulder, a recoverable slope, a non-recoverable slope, and/or a clear run-out area."

clear zone illustration
Photo Source: FHWA

Q: How wide is the clear zone?

A: The desired width is dependent upon the traffic volumes and speeds and roadside geometry. As an example, a roadway with 400 vehicles per day, a posted speed limit of 55 MPH and 4:1 side slopes would have a calculated clear zone of 23 feet, per figure 3.1b; RDG.

Q: Is additional information available on these topics?

A: Yes, the following resources provide additional information on sign supports and crashworthiness.

If you are using a proprietary product then it should have an acceptance letter listed here.