The only types of sign-support systems that should be used are those that have been approved for use by the FHWA. The following concerns should be addressed in the selection of an appropriate single-sign-support system:
• The specifications for support size provided by many states provide information on the maximum sign panel area to be mounted on the support. The shape of the sign as well as the area should be considered when determining the type and number of supports required. For example, a 5-ft X 2-ft (1525-mm X 610-mm) guide sign will have less area than a 4-ft X 4-ft (1220-mm X 1220-mm) warning sign. The wide dimension of the guide sign, however, will result in excessive vibration from wind loads if it is placed on a single sign support without bracing. As a general rule, signs over 40 in (1000 mm) wide should be placed on multiple supports.
• Sign-support systems that are not placed in concrete foundations perform better in strong soils than in weak soils, such as sand. When the system is directly placed in weak soils, an anchor plate, a proper concrete footing, or embedment to a greater depth than used for strong soils may be required. This will hold the post firmly in the ground, preventing rotation due to wind loads, and help ensure proper operation during impact.
• The embedment depth is important for proper sign assembly operation. One-piece sign assemblies will pull out of the ground if not buried sufficiently deep. If buried too deep, it is difficult to remove the buried segment. Similarly, proper embedment depth for assemblies that use an anchor piece is important to prevent damage to the anchor piece on impact and to prevent rotation due to wind loads. The proper embedment depth varies by type of support system.
• Do not use sign-support sizes larger than required to support the sign or larger than approved for single-support types. For example, a slip base assembly should be used rather than a 6-lb/ft (9-kg/m) U-channel post.
• Do not combine supports, such as square tube inside of pipe, or double the supports, such as back-to-back U-channels.
• Do not use diagonal bracing to strengthen a damaged or improperly designed support system.
• Sign-support assemblies are categorized as unidirectional, bidirectional, and multidirectional. Unidirectional supports will function properly only when impacted from one direction, and bidirectional, from two directions. Multidirectional supports will function properly when impacted from any direction.
• The same type of support post can be configured to operate in different ways upon impact. For example, the U-channel post is basically a unidirectional, base-bending support when buried directly in the ground. It can also be spliced to an anchor piece to provide breakaway characteristics or installed with a frangible coupling to provide multidirectional capability.
• Whenever an anchor system design is used, the anchor stub should not extend more than 4 in (100 mm) above the ground. Extensions above the ground more than this can snag the vehicle undercarriage.
• A minimum mounting height of 9 ft (2740 mm) from the ground to the top of the sign panel is recommended for all single-sign-support installations. Mounting the signs at this minimum height will reduce the possibility of windshield penetration by a sign that bends or yields into the vehicle upon impact.
