Breakaway supports for signs, luminaries, motorist aid callboxes, traffic signals, mailboxes, and other features placed alongside the road are designed to break or yield when struck by a vehicle. The FHWA has adopted the testing parameters and criteria in the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals for determining acceptable breakaway performance. AASHTO issued new editions to this specification in 1985 and 1994.
The 1994 AASHTO edition changed the maximum acceptable velocity change to 16.0 feet per second and converted all values to SI units. The 1993 NCHRP Report 350 contains detailed guidance for crash testing of all highway features. Note that breakaway support hardware acceptable under either the 1985 or 1994 editions of the AASHTO specifications are acceptable under the NCHRP Report 350 testing guidelines. In 2009, AASHTO published the Manual for Assessing Safety Hardware (MASH) which is to be used for crash testing of new devices.
Following adoption of the 1985 AASHTO crash-test specifications, the FHWA began issuing acceptance letters to manufacturers of breakaway support systems acknowledging that the devices tested successfully according to the parameters and performed satisfactorily. Typically, the acceptance letters describe the device tested and include a drawing of the device, test results, and information on limitations on use of the device, such as the weight of the system tested or the soil in which it is acceptable. Also, FHWA policy requires the use of crash-worthy hardware within the clear zone of all NHS routes, regardless of the ownership of the roadway or the source of project funding.
Three lists of breakaway hardware are: