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 ¼ 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."
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.
- Minnesota Manual on Uniform Traffic Control Devices
- FHWA Roadway Departure Crash Severity Reduction Countermeasures
- Maintenance of Sign and Sign Supports - A Guide for Local Highway and Street Maintenance Personnel
If you are using a proprietary sign support product then it should have an eligibility letter listed on the Sign Support, Mailboxes & Delineators website.