Design and Testing of a Break Away Mount for Cluster Box Unit (CBU) and Neighborhood Delivery and Collection Box Unit (NDCBU) (Research Project 405461)

Neighborhood Delivery and Collection Box Units (NDCBU) and Cluster Box Units (CBU) present a special challenge to the highway engineer due to their large physical size, mass, and tall mounting height. Efforts expended in the past by TTI researchers and others have demonstrated the hazards of mounting various types of mailbox installations within the clear zone. In addition, TTI and the Texas Department of Transportation, having performed full-scale crash tests, concluded that the rigid mounting of CBU's using 13 mm (0.5 in) studs or bolts into a concrete footing was unacceptable for installation in the clear zone of high-speed roadways. To this end, TTI concluded from the study that impact performance of CBU type installations could be improved by the inclusion of a breakaway feature(s) such as a slip-base, breakaway welds at the post to base connection, a weakened support, or a combination of these. In addition, even with a frangible or slip connection, the CBU mounted at windshield height presents an additional hazard that must be addressed.|The United States Postal Service (USPS), recognizing that a potential hazard to the errant motorist may exist where CBU's are installed, contracted with Texas Transportation Institute (TTI) to: (1) evaluate the potential crash performance of pedestal mounted Neighborhood Delivery and Collection Box Units (NDCBU) and Cluster Box Units (CBU) currently being used in the field, (2) design a prototype retrofit breakaway mount for these units, and (3) provide initial laboratory testing of the new frangible mounting system. The end objective of the study was to investigate the development of a frangible retrofit system for mounting existing and new CBU installations that would satisfy the crash performance criteria presented in the NCHRP Report 350 and the 1994 AASHTO "Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals."