Sign Assemblies

The sign panel, the support, and the embedment or anchorage system are the three components of a sign assembly. Each component contributes to the effectiveness, structural adequacy, and safety upon impact of the device. The sign assembly must be structurally adequate to withstand its own weight and the wind and ice loads subjected to the sign panel. In some northern climates, this requirement includes the forces created by snow ejected by snowblowers or the lateral forces resulting from snowplow activity. The majority of the design guides for each state contain recommendations on the size, number, and type of support required in different regions of the state. These guidelines are based on the size of the sign panel and the recurrent wind intensity. Average wind loads for 10-, 25-, and 50-year recurrence intervals are also contained in AASHTO’s Standard Specification for Structural Supports for Highway Signs [12].

7.2.1 Sign-Support Considerations

There are a variety of systems used to support ground-mounted traffic signs. These systems were often categorized by whether they were intended to support small or large signs. Small signs were arbitrarily defined as those having a total panel area of less than 50 ft2 (4.7 m2) [17]. This designation is, however, arbitrary and not effective in identifying the characteristics of the support used. An alternative method of catego­rizing sign types is by designating them as single — or multiple-mount systems. Multiple mounts include two or more supports that are separated by 7 ft (2100 mm) or more. Sign panels supported by a single support or by multiple supports less than 7 ft (2100 mm) apart are considered single mounts. The separation criterion allows for the possibility that a vehicle, leaving the roadway at an angle, can impact more than one support. Signs supported by more than one support, in addition to being separated by more than 7 ft (2100 mm), must also be designed for each support to independently release from the sign panel. Multiple-support systems, therefore, must have sign panels with sufficient torsional strength to ensure proper release from the impacted support while remaining upright on the support(s) that were not impacted. This also requires that the remaining support(s) have sufficient strength properties to prevent the sign panel from breaking loose and entering the passenger compartment or becoming a projectile.

Metal supports that yield upon impact have been used for many years to provide effective economical supports for traffic signs. The U-channel post design is the most widely used support for both single — and multiple-support designs [17]. Yielding supports are designed to bend at the base and have no built-in breakaway or weakened design. Systems in this category include the full-length steel U-channel, aluminum shapes, alu­minum X-posts, and standard steel pipes. For successful impact performance, the support must bend and lie down or fracture without causing a change in vehicle velocity of more than 10 mi/h (5 m/s). Tests have shown that supports that fracture offer much less impact resistance, especially at high-speed impacts, than yielding supports of equal size.

The impact behavior of base-bending supports depends upon a number of complex variables including cross-sectional shape, mechanical properties, energy-absorption capabilities under dynamic loading, chemical composition, type of embedment, and characteristics of the embedment soil. The wide number of variables related to the properties of the support itself require that full-scale crash testing be performed to evaluate the impact behavior of base-bending supports. Tests are performed on cate­gories of support types that need to be specified during their purchase. For example, U-channel posts, while of the same shape, will have different impact characteristics depending upon their unit weight and whether they are cold-rolled or hot-shaped.

The impact performance of base-bending supports depends upon the interaction between the structure and the soil in which it is embedded. Soil conditions vary drasti­cally with location, even within small geographic locations. Due to this variability, NCHRP 350 has established standard soil conditions (previously referred to as “strong soil”) and weak soil for testing. Weak soil consists of relatively fine aggregates that provide less resistance to lateral movement than that provided by a standard soil.

The rules on weak soil versus strong soil are, however, in question. The FHWA has insisted that yielding supports be qualified in both soils in order to be eligible for federal aid. However, recently completed crash testing yielded very few acceptable supports in weak soil. FHWA considers that it may be too restrictive to forbid all use of those supports that failed in weak soil. The standard soil in NCHRP Report 350 is the “strong soil.” If a state has potential sites where the device will be installed in weak soils and believes that the device may not behave as well as in strong soil, then weak soil testing is called for. Otherwise, a device that has been found acceptable only in strong soil may be used only in strong soil.

The proper performance of some base-bending supports requires that they do not pull out of the soil upon low-speed impact. Placing these base-bending devices in weak soil, when they have been approved for use only in standard soil, or at an improper embedment depth will not provide acceptable low-speed performance. If the device was installed on a narrow median, for example, it can pull out of the ground upon impact and become a lethal trajectory to opposing traffic. Consideration must be given to the soil acceptance criteria of the post planned for use, the soil condition pre­sent, sign location, and the safety performance needs of the sign assembly.

Breakaway supports are designed to separate from the anchor base upon impact. Breakaway designs include supports with frangible couplings, supports with weakened sections, bolted sections, and slip base designs. Breakaway supports are classified by their ability to properly separate from the base upon impact from one direction (unidi­rectional) or from any direction (multidirectional). Large signs, requiring multiple supports separated by 7 ft (2100 mm) or more, often use a hinged breakaway mecha­nism with a horizontal slip base. The use of slotted hinge plates, on both sides of the upper beam, and a horizontal slip base results in proper device function from either the front or the back. The action of the hinged breakaway is illustrated in Fig. 7.2.

In addition to the yielding and breakaway sign support, overhead and fixed-base support systems may be used. Overhead sign support systems include the use of exist­ing structures, such as bridges, that span the traffic lanes. Fixed-base support systems include those that do not yield or break away upon impact. Fixed-base systems are made of materials that will not fracture upon impact and are firmly embedded in or rigidly attached to a foundation. Fixed-base systems are often used to support overhead signs on roadway facilities with three or more lanes or for traffic signal supports. The large mass of these support systems and the potential safety consequences of the sys­tems falling to the ground necessitate a fixed-base design. Fixed-base systems are rigid obstacles and should not be used in the clear zone area unless shielded by a barrier.

The total combination of support systems and methods of embedment is large. Considering the following factors can assist in selecting the most appropriate sign support system:

• Large ground-mounted signs can be located 50 ft (15 m) or more from the edge of pavement on high-speed facilities. These substantial lateral clearances increase the roadside recovery zone while still meeting motorist viewing needs.

• The performance of any sign assembly is influenced by the surrounding terrain. Terrain that will cause the vehicle to impact the sign assembly at a higher or lower point than the design impact height can cause unpredictable and often hazardous results.

• The height of post-mounted signs is determined by drivers’ need of a legible mes­sage and by the functional requirements of the support system. A breakaway sup­port system designed with a hinge, for example, will not function properly if the sign is mounted so low on the support system as to interfere with the hinge action.

• Efforts should be exerted to keep the top of the sign panel at a height of 9 ft (2700 mm). Placing the sign at this height reduces the possibility that the top of the sign will break the windshield and intrude into the passenger compartment during impact. If the top of the sign panel is at least 9 ft (2700 mm) high, then the sign will hit the vehicle’s roof and reduce the probability of vehicle intrusion. The majority of signs that meet the MUTCD standards for clearance to the bottom of the sign will also meet the minimum height to the top of the sign panel. Exceptions to this include rural installations with mounting heights less than 7 ft (2130 mm) to the bottom of the sign with sign panels less than 4 ft (1200 mm) in size.

• Traffic signs should not be considered permanent solutions to inappropriate or haz­ardous roadway conditions. Installing a warning sign, for example, to warn of a shoulder dropoff does not eliminate the dropoff problem and presents an additional fixed object.