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Frequently Asked Questions:
Q: Why should I care about crashworthy sign supports and mailboxes?
Q: How do I know if my sign posts are crashworthy?
Q: How wide is the clear zone?
Q: Is additional information available on these topics?
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 the following link: FHWA GUIDE: MAINTENANCE OF SIGNS & SIGN SUPPORTS (2010).
- 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.
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; Roadside Design Guide.
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 (MN MUTCD):
http://www.dot.state.mn.us/trafficeng/publ/mutcd/index.html
FHWA has a site dedicated to breakaway features for sign supports. It can be found
by going to:
http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/ctrmeasures/breakaway/
The booklet “Maintenance of Signs and Sign Supports” can be found at:
http://safety.fhwa.dot.gov/local_rural/training/fhwasa09025/
If you are using a proprietary product then it should have an acceptance letter listed at:
http://safety.fhwa.dot.gov/roadway_dept/policy_guide/road_hardware/breakaway/signsupports.cfm
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