Multimount sign supports have two or more support posts of breakaway design. The breakaway mechanism is either a fracture or a slip base type. Fracture mechanisms consist of frangible couplers or frangible one-piece posts.
7.8.1 One-Piece Multiple-Mount Sign Supports
Direct burial assemblies that are approved for use include dual 3-lb/ft (4.5-kg/m) U-channel and dual 4-in X 4-in (90-mm X 90-mm) wooden posts that have been modified with two 1.5-in (38-mm) holes placed at 4 and 18 in (100 and 450 mm) above the ground line. Other than these exceptions, multiple-mount sign supports require the use of anchor pieces, sleeves, slip bases, or frangible couplings for acceptable impact performance. Only those devices approved for use by the FHWA should be used for multiple-mount supports. Since more than one support can be simultaneously struck by an errant vehicle, it is important that proper installation procedures be followed. Figure 7.46 illustrates the holes drilled in a direct burial wooden post to provide the weakened cross-section required for multiple wooden post installation.
