An important element of a safe highway environment is the proper construction and maintenance of traffic signs. Good designs and the best of materials will not be effective in reducing accident potential or severity if the traffic signs are improperly placed or installed. This requires that field crews be knowledgeable of proper installation techniques and that they report and correct any possible problems instead of merely placing the signs at the roadside. (See also Art. 7.1.2.)
Important considerations for proper placement include the following:
• Warning signs should be placed sufficiently in advance that the driver has adequate time to perceive, identify, decide, and perform any necessary maneuver. A guide for the placement distance of warning signs is contained in MUTCD [2].
• The MUTCD provides guidelines on the height and lateral placement of typical sign installations. Signs installed on the side of the road in rural districts should be at least 5 ft (1500 mm) measured from the bottom of the sign to the near edge of pavement. In urban districts or where parking or pedestrian movements occur, the clearance to the bottom of the sign should be at least 7 ft (2100 mm). Ground-mounted signs on freeways and expressways should be at least 7 ft (2100 mm) above the level of the pavement edge and have the minimum lateral offset of 12 ft (3700 mm) from the edge of the traveled way. If a shoulder wider than 6 ft (1800 mm) exists, the minimum lateral offset for ground-mounted signs should be 6 ft (1800 mm) from the edge of the shoulder.
• All sign assemblies located within the traversable area must be capable of giving way safely upon impact. This requires that the maximum vehicle deceleration does not exceed 16 ft/s (5 m/s) and that the sign assembly does not protrude into the passenger compartment [14].
• Sign supports installed with anchor systems must have a maximum height of 4 in (100 mm) from ground level to the topmost part of the anchor.
• Most sign-support assemblies are designed to function properly when impacted at bumper height, typically about 20 in (500 mm) above the ground. If impacted at a higher point, the assembly may bind at the planned shear point resulting in nonactivation of the breakaway mechanism. For this reason, it is critical that breakaway sign assemblies not be located near ditches or on steep slopes or other locations where the vehicle can become partially airborne at the time of impact.
• Sign supports should not be placed in ditches. The water in the ditch can erode the soil around the base of the support, cause premature deterioration of the post, and freeze, resulting in unpredictable performance during impact. The ditch can also act as a guideway that directs errant vehicles into the sign assembly.
• Sign-support assemblies are tested in both strong and weak soils. Supports that are designed to yield, or fracture, upon impact generally perform better in strong soil. Strong soil holds the buried portion in position, providing sufficient resistance for the sign support to break near ground level. Weak soils do not provide this resistance, but permit movement within the ground and unpredictable results. Yielding or fracturing supports that are embedded less than 40 in (1000 mm) in weak soil will often pull out of the soil. While this may provide acceptable impact performance, the force of the wind and ice loads may cause the sign assembly to rotate or fall down. The actual soil type that is present may not be known until the start of installation. Weak soils are those that offer relatively little resistance to driving the signpost. If weak soils are encountered, there are measures that can be taken to maintain sign orientation in the face of environmental loads and still result in proper operation during impact. These include embedding the signpost to 40 in (1000 mm), and the use of anchor plates, concrete footings, and commercially available anchor systems [34].
• Single-sign-support systems are designed to operate safely when only one support is struck upon impact. Tests have shown that an errant vehicle, leaving the roadway at an angle, can impact more than one support if supports are not separated by more than 7 ft (2100 mm). This separation applies to other fixed objects as well as signposts. For example, a 3-in-diameter (75-mm) tree is sufficiently small to provide acceptable impact performance. Installing a sign support 6.5 ft (2000 mm) from this tree, however, can result in an errant vehicle’s impacting both the tree and the signpost. The combined effect of the tree and sign can provide unacceptable impact performance.
• Multiple-mount sign supports are required to support sign panels that are too large to withstand wind and ice loads with the use of only one support. Multiple-mount sign supports are designed to provide acceptable performance upon impact when the supports are placed 7 ft (2100 mm) or closer to each other. This close spacing results in the possibility that a vehicle leaving the roadway at an angle will impact two or more supports simultaneously. This possibility means that some supports approved for use in a single-support system are not approved for multimount designs. Support configurations that have not been approved for use as multiple — mount systems should not be used for multimount sign supports until they have been approved by the FHWA.
• Study the traffic patterns and surrounding geometrics prior to installing any sign. If the sign assembly can be expected to be struck from more than one direction, then a unidirectional slip base design is an improper choice. Two-lane rural roadways should use roadside supports that function safely when impacted from two directions. Installations on freeways, with wide medians or positive median barriers, can be expected to be impacted from only one direction.
• In summary, be aware of what is required for sign installations to function properly for both the environmental loads and vehicle impact. Do not install a device simply because it has been specified on the sign order. The actual site conditions may have been unknown, or different from what was expected by the designer who specified the type of sign assembly. If problems are identified, contact a supervisor to determine if changes should be made.
