The following guidelines should be followed for multiple-sign-support construction:
• Multimount sign supports are designed to function properly when more than one support is struck by an errant vehicle. There will be occasions, however, where only one support will be impacted. When this occurs, it is necessary that the sign panel be properly fastened and have sufficient rigidity so that the post(s) that are not impacted will support the sign panel, preventing intrusion into the vehicle.
• The hinge should be located at least 7 ft (2100 mm) above the ground to prevent the upper section of the support from penetrating the windshield.
• No portion of the primary sign, additional signs, or bracing should be attached to the supports below the hinges. Fastening below the hinge will interfere with the breakaway performance of the support post. Signs that are mounted to the primary sign panel and are less than 7 ft (2100 mm) above the ground can intrude into the passenger compartment even when the hinge operates correctly. Supplemental sign panels should not, therefore, be less than 7 ft (2100 mm) above the ground.
• Two posts within a 7-ft (2100-mm) path should each have a mass that does not exceed 18 lb/ft (27 kg/m).
• Slip base mechanisms must be constructed with the proper size bolts and washers. Oversized bolts can result in bending between the upper and lower base plates. Washers that are too thin can deform into the slots and bind the plates together.
• The torque specifications must be followed when assembling slip bases and hinges. With insufficient torque, wind and ice loads can cause the bolts to become loose, with subsequent “blowdown” from hinge release or “walking” at the slip base. Applying too much torque can result in binding between the mating surfaces, with subsequent improper operation upon impact.
• Crash tests, performed on level terrain, indicate that breakaway designs perform satisfactorily upon impact. When installed on slopes, however, there is the possibility that they may not function as planned. This is due to the slope’s changing the trajectory of the impacting vehicle from the test conditions achieved with level ground. Multiple-mount signs should be installed on level ground when possible and outside the clear zone, in a location where they will be least likely to be hit. Some state agencies routinely require multimount signs to be installed 40 ft (12 m) or more from the edge of the traveled way.
• Follow the installation plans of multiple-mount supports for both construction and maintenance. Do not make temporary maintenance repairs using wrong size bolts or shear plates. Temporary repairs often become permanent, or at any rate can be subjected to an impact prior to correction.
• Do not install any sign supports in a ditch line. The water funneled in the ditch will cause premature corrosion and can freeze, preventing proper operation. The ditch can also channel errant vehicles and guide them into the support.
• Multiple-mount sign support systems are often classified as dual and triple installations. This classification refers to the number of posts permitted within a 7-ft (2100-mm) radius. Approval of support types for dual installation, for example, indicates that no more than two of these supports are permitted within a 7-ft (2100-mm) radius of each other. Acceptable impact performance can be achieved by reducing, but never increasing, the number of supports. A support type approved for dual use can be installed as a single-mount post but not as a triple installation.
• Multimount supports installed with slip base and/or frangible coupler designs must have a maximum height of 4 in (100 mm), over a span of 5 ft (1.5 m), from the ground to the topmost part of the anchor. This is necessary to prevent the anchor piece from snagging the undercarriage of impacting vehicle.
• Each post of a hinge design should be fabricated from a continuous piece of material. The holes for the friction and hinge plates should be drilled and sections match — marked before cutting and weatherproofing. The match marks must be visible after weatherproofing.
• Supports, posts, and anchor pieces should be fabricated and assembled in a shop to ensure proper alignment and match of base plates. Any dismantling that may be required necessitates the placement of match marks to ensure reassembly in the original manner.
• Each post should be installed as a unit to ensure proper alignment of the post and anchor piece assemblies.
• Proper functioning of the slip base feature requires that the interior washers, between the post slip plate and the anchor piece slip plate, transfer the bearing pressures equally. After assembly, the upper and lower slip plates should have a clearance between them of at least % in (3 mm) but not in excess of 34 in (7 mm).
• All bolts for attaching the signs to the stiffeners should be %s in (8 mm) placed in bolt holes of % in (10 mm). Flat washers should be used beneath the head of hex head bolts. Fiber washers should be used beneath the head of carriage bolts to prevent possible damage to the reflective sheeting when tightening. All bolts should be sufficiently long to allow the bolt to extend beyond the nut when tightened correctly.
