U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Q: SHOULD WE USE BREAKAWAY BASES FOR SIGN AND LIGHT POLES MOUNTED ON CONCRETE MEDIAN BARRIERS?
A. No, breakaway bases should not be used. Mounting any pole on top of a median barrier should be avoided because trucks will lean over the barrier upon impact and hit whatever is on top. A rigid pole may or may not break off, but there is no safety advantage in making it easier for the pole to break away and fly into the opposing travel lanes.
The potential for a pole being struck by the box of the truck can be minimized by making the barrier wider. If you transition to a vertical face and/or taper the width of the barrier you can provide additional offset to the pole. The point is to minimize the potential for broken poles to fly into the opposite roadway. Larger passenger vehicles as well as their occupants may contact objects on top of barriers under severe impact conditions.
Work zone signs may be mounted on barriers if you use roll up signs on fiberglass supports as they have less potential for causing serious damage.
Q: Can we still use pendulum tests to assess breakaway performance of sign and lighting poles?
A. The status of pendulum testing under MASH is undetermined – if the bases can be considered modifications of bases tested under Report 350 then we will agree to review pendulum testing done under 350 criteria (until 1/1/11). If they are totally new designs, then they need to be tested under MASH. Whatever test method you use must evaluate the roof crush and windshield damage as required in MASH. Based on experience in reviewing both full scale testing and pendulum tests of breakaway supports, we do not see how pendulum testing can answer those questions. The old “rule of thumb” limiting luminaire supports to 1000 pounds cannot be relied upon because vehicle structures have changed and MASH now requires the pickup truck be used. Crash tests of luminaire supports with pickups is very limited (we are not aware of any such tests) so we have no way to assess the potential for roof crush except through full-scale testing.
Q: Should we use omni-directional breakaway bases everywhere?
A. Crashworthy omni-directional bases are designed to meet NCHRP Report 350 criteria regardless of which direction they are struck. They are required when installing signs and other breakaway hardware near intersections (including expressway ramps ending on local roads and in roundabouts) and preferred on other undivided facilities. Uni-directional bases (i.e., 4-bolt slip bases) should only be used along divided highways where impacts are limited to the direction of travel of the roadway they are next to.
Q: SHOULD TREES, POLES, OR OTHER FIXED OBJECTS BE ALLOWED IN A ROUNDABOUT?
A. Typically it is desirable to have some type of streetscape/landscaping installed within the central island of the roundabout to increase conspicuity, break the headlight glare of oncoming vehicles, and promote lower speeds through the roundabout. However, care is needed when considering landscaping that introduces fixed objects that may pose a safety concern for errant vehicles. In most cases, fixed objects should be minimized in the perimeter area of the central island where there is a higher risk of the objects being struck.
Small trees, structures and statues placed within the inner portion of the central island may offer the benefit of helping obscure the line of sight straight through the roundabout. The perimeter portion of the central island may be landscaped with low-level vegetation so that stopping sight distances are maintained for vehicles within the circulatory roadway and at the entrance lines of the roundabout.
As for "clear zone", consideration of the roundabout context and expected operating speeds should be considered consistent with the principles outlined in the updated AASHTO Roadside Design Guide for urban conditions.
Q: WHAT IS THE JANUARY 1, 2013 DEADLINE FOR BREAKAWAY SUPPORTS?
A. All sign supports within the clear zone of highways signed at speeds of 50 mph or greater must be mounted on breakaway supports or be shielded with a barrier by January 1, 2013, per the MUTCD.
Breakaway supports meeting the current crash test criteria* have been required on all Federal-Aid Projects since 1990. (Since the ISTEA of 1991 that requirement has applied to all projects on the NHS regardless of funding source.) The 2000 edition of the MUTCD made breakaway supports mandatory for signs within the clear zone of all roads open to public travel in the United States. This requires that all new sign installations be on breakaway supports. While shielding with guardrail is an option, use of breakaway supports is preferred. The 2003 edition of the MUTCD set a 10-year implementation period to retrofit sign supports on highways signed at 50 mph or greater.
* The ‘current crash test criteria’ were established in 1985 with the publication of the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. The 1993 NCHRP Report 350 incorporated the same test and evaluation criteria in metric form. Because the metric conversion in Report 350 resulted in tests that were slightly more liberal than the 1985 Specifications, all breakaway testing done between 1985 and 1993 is considered acceptable under NCHRP Report 350. The 2009 MASH adds the pass/fail criteria for windshield damage and roof crush, and requires testing with the 5000# pickup truck, but maintains test and velocity change evaluation criteria for the small car equivalent to that adopted in 1985.
Q: WE WANT TO ADD LIGHTS, A BATTERY, AND A SOLAR PANEL TO OUR SCHOOL ZONE SIGN. DOES THE COMBINATION HAVE TO BE CRASH TESTED?
A. There are four factors that determine the acceptability of breakaway supports:
Safe placement of these types of devices on the sign also depends on the structure of the sign, the sign height, the type of vehicle impacting the sign, and the breakaway nature of the sign support when it is impacted. The conditions outlined above assume the sign pole is rigid and that the pole itself will not deform upon impact. Also the breakaway feature must be a slip base, frangible coupling system, or a cast aluminum transformer base – “base bending or yielding” systems such as u-channel posts, perforated square steel tube posts, or composite posts require full scale crash testing.
Q: Is it OK to add a decorative shell or casting to a breakaway light pole to make it more aesthetic?
A. Adding decorative hardware to a breakaway luminaire support might affect proper performance. Thus, aesthetic or decorative modifications should be crash tested.