Category HIGHWAY ENGINEERING HANDBOOK

Wooden support posts are available in shaped sizes, as engineered products, and as timber posts. The shaped sizes are described by their nominal dimensions, such as 4 in X 4 in (100 mm X 100 mm). This is their size prior to the surfacing required to provide smooth and straight posts. Their actual size is typically less than the nominal size. A 4-in X 4-in (100-mm X 100-mm) post will therefore have an actual size of 3.5 in X 3.5 in (90 mm X 90 mm). The engineered products are made from laminated or pressure-glued wood and nonwood recycled products. Timber posts are round in shape.

All wooden posts are of breakaway design, with the intended fracture of the post near the base and less than 4 in (100 mm) above the ground. The post features that influence fracture include the size of the post, effective cross-sectional area, embedment depth, type of soil, and the species of wood. The majority of wood post tests have been conducted using grade 2 southern yellow pine posts.

Shaped Wood Posts. The most common size wood post used for single sign installations is the 3.5-in X 3.5-in (90-mm X 90-mm) support. This support should be buried directly in strong soil to a depth of at least 36 in (920 mm). The cross-sectional area of this post is sufficiently small that drilled holes are not needed to provide a weakened section.

A 3.5-in X 6-in (90-mm X 140-mm) post installed in strong soil will provide acceptable performance upon impact without reducing the cross-section. Tests have shown, however, that the 3.5-in X 6-in (90-mm X 140-mm), when installed in loose or

sandy soil, is unacceptable when impacted by a small vehicle. Single wood posts of that size installed in weak soil should be modified with 1.5-in-diameter (40-mm) holes to be crashworthy. The holes should be centered at 4 and 18 in (100 and 460 mm) above the ground line and perpendicular to the roadway centerline.

Table 7.3 provides the modifications required for acceptable performance of various shaped wood post sizes. The holes in each case are drilled at 4 and 18 in (100 and 460 mm) above the ground line and perpendicular to the roadway centerline.

Typical details for installation of shaped wood posts by direct burial and concrete methods are presented in Fig. 7.12. The 6-in X 6-in (140-mm X 140-mm) post should be set in unreinforced concrete to help ensure that the post fractures upon impact. To make it easier to remove a broken stub, a post can be wrapped with 0.5-in-thick (13-mm) Styrofoam prior to filling with concrete.

90 50 0ІА HOLES

2750

CROSS SECTION 90×90 OR 90X140

STYROFOAM

910

Some states have used larger shaped wooden posts, such as 6 in X 8 in (140 mm X 215 mm), with appropriately sized holes to reduce the cross-section area. These holes provide a weak section that appears acceptable, but the increased mass of these posts and lack of testing result in unpredictable impact performance. Shaped wooden posts larger than those that have been crash-tested should not be used. If larger posts are required, then a multiple-post configuration, slip base design, or other alternatives should be used.

Engineered Wood Posts. A number of relatively new products have been developed for use as sign supports. These include engineered wood product posts made from recycled plastics and wood chips, and laminated veneer lumber posts. The Microllam laminated posts in 8 in X 8 in (200 mm X 200 mm) and in 15 in X 8 in (380 mm X 200 mm) have been accepted for use. These posts, manufactured by the Trus Joist MacMillan Corporation, have a wall thickness of 1.25 in (32 mm) and mitered 45° corners. The post is placed in predrilled holes and backfilled. The posts require four 1-in-diameter (25-mm) holes drilled on the two sides parallel to the direction of travel. Two of the holes are at 3 in (76 mm), and the other two holes are at 18 in (457 mm) above ground height. A saw cut parallel to the ground that connects each set of holes is required.

Timber Poles. The majority of wooden sign-support systems consist of square or rectangular shapes. However, round timber poles, up to 7.5 in (190 mm) in diameter of southern pine, grade 2, have been accepted for use by the FHWA [25, 26]. The acceptable sizes and required holes to provide acceptable breakaway performance are presented in Table 7.4.

The steel U-channel support is the most common type of single sign support used in the United States [19]. The steel U-channel is a unidirectional support available in different sizes and stiffnesses from a variety of manufacturers. The most popular steel U-channel sizes are 2, 2.5, 3, and 4 lb/ft (3, 3.7, 4.5, and 6.0 kg/m) (weight is prior to making the fastening holes). The channel is constructed with %-in (9.5-mm) holes on 1-in (25.4-mm) centers to eliminate the need for drilling to mount the sign panel. The posts are available with baked alkyd resin, with gloss enamel paint, or hot-dipped galvanized to inhibit corrosion. The stiffness of U-channel posts is a function of the material from which they are made, and the method by which they are shaped. The literature refers to billet steel or rail steel as the material from which U-channel is constructed. Rail steel is old railroad track—which has a high carbon content—that has been rerolled into the U-channel shape. The high carbon content results in a steel that is strong but relatively brittle. Billet steel is newly formulated steel. The most common grade of billet steel is A36, which is a relatively low-carbon “mild” steel, but billet steel can be manufactured with sufficient carbon to equal or exceed the strength characteristics of rail steel. For years the FHWA required “rerolled rail steel” instead of billet steel, since such a speci­fication helped ensure a high carbon content. High-carbon billet steel U-channel posts are available from manufacturers, but most state specifications still refer to “rail steel.”

Crash tests indicate that, under some test conditions, high-carbon steel U-channel sign posts perform differently from those made of steel having a lower carbon content. The reason for this performance difference is that high-carbon steel posts, because of their low fracture toughness, break under high-speed impacts [22]. Lower-carbon steel posts bend and shape themselves to the front of the vehicle, thereby forming a tether­ing hook. Billet steel posts that have carbon content similar to that found in rail steel posts match the performance of the rail steel posts when crash-tested.

Table 7.2 presents the maximum allowable sign area for one-piece rail steel U-channel installations. The table is for a maximum allowable pressure resulting from a 70-mi/h (113-km/h) wind velocity. State guidelines should be followed for the expected wind veloc­ities for different regions of the state and to obtain support sizes for other wind velocities.

Base-Bending Installation. One-piece base-bending U-channel post installations are usually obtained by driving the post directly into the ground with a sledge hammer, manual post driver, or air-operated post driver. Drive caps should be used to protect the top end of the post while it is being driven into the ground. U-channel posts should not be encased in concrete unless a breakaway design is used. Typical embedment depth is 3 ft (920 mm), and for ease in removing damaged posts, the driven depth should generally not exceed 3.5 ft (1070 mm). The patented RIB-BAK design of a U-channel has a ribbed back and flange. This design provides extra strength, a flush back-to-back sign-mounting surface, and a ridge for mounting channel locking clips. An alternative to the direct burial system is the V-loc anchor from Foresight Industries. The V-loc anchor is currently the only alternative method of anchoring an unspliced U-channel post that has been found acceptable by FHWA. This anchor system uses locking inserts to hold the U-channel securely into the V-shaped anchor piece. Upon impact, the post will bend at the ground line and may pull completely out of the V-loc anchor.

Breakaway Installations. The repair and performance of large U-channel posts can be eased by using a breakaway design. Breakaway design in U-channel installations is obtained by developing splices at ground level. The splice consists of attaching the signpost to an anchor piece that is embedded in the soil or a concrete foundation. Splice designs include the Eze-Erect by Franklin Steel, the Minute-Man by Marion

TRAFFIC

FIGURE 7.8 Breakaway devices for U-channel posts. (a) Eze-Erect splice joint. (b) Details of Eze-Erect splice. (c) Minute-Man coupler. Dimensions shown as mm; 100 mm = 4 in.

Steel, and various lap designs [23]. The intent of the splice designs is for the splice to fail upon impact. The commercially available splices are designed so that the signpost remains attached to the embedded anchor piece and passes beneath the impacting vehicle. This is accomplished by designing the splice device to partially fracture or to completely fracture a frangible coupling. To prevent vehicle snagging, the anchor piece should not extend more than 4 in (100 mm) above the ground. Two commercially avail­able breakaway designs are presented in Fig. 7.8. The Minute-Man consists of a frangible coupling with a backup plate, to hold the anchor and sign-support pieces together. The Minute-Man coupler makes the U-channel a multidirectional support system.

The generic splice (Fig. 7.9) does not require special hardware [24]. It is acceptable for use on 4-lb/ft (6-kg/m) U-channel, or less, installed in strong soil. The generic splice consists of an overlap of 6 in (150 mm) and uses two %s-in (8-mm) bolts spaced 4 in (100 mm) center to center. Spacers, %; in (8 mm) thick, are used to separate the U – channel signpost and the anchor piece. The spacer must be strong enough to transfer the load between the webs of the signpost and the anchor piece. The signpost should be mounted behind the stub.

The anchor piece of all breakaway devices should be the same size as the signpost and must not extend more than 4 in (100 mm) above the ground. A splice configuration, as in Fig. 7.10, does not provide protection for the anchor and increases the probability of snagging or of the sign’s entering the passenger compartment. Breakaway devices improve the safety characteristics of the post and generally reduce maintenance costs. They should always be used when the sign support is placed in concrete areas. If the sign can be impacted from different directions, then a breakaway device similar to that shown in Fig. 7.8 should be used. Splicing the signpost to the anchor piece with bolts, with or without the splice breakaway device of Fig. 7.8a, does not make the U-channel support a multidirectional sign support.

Mounting Concerns. The U-channel post is approved for use in strong soils when impacted from a frontal direction. Installing the support in weak soils or in locations where

BASE POST

FIGURE 7.9 Details of generic splice configuration. Dimensions shown as mm. Conversions: 8 mm = %5 in, 25 mm = 1 in, 100 mm = 4 in, 150 mm = 6 in, 950 mm = 37 in.

it can be impacted from more than one direction requires more than direct burial to make the U-channel perform as required. If the U-channel is installed in weak soil, an anchor plate, similar to that shown in Fig. 7.11, can be used to hold the sign in its proper position and to help ensure proper operation upon impact. In addition, the generic splice can allow the signpost to separate from the base. The possible consequences of this separation, and the trajectory of the sign assembly, should be considered prior to use of the generic splice.

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.

Sign assemblies consist of four components:

• The sign panel on which the message is displayed

• The signpost

• Mounting hardware and fasteners

• The base for the post

Sign Panels. The majority of sign panels in use today are made from sheet alu­minum stock [19]. The thickness of the stock varies depending upon the sign size but is generally not less than 0.16 in (4.0 mm). Plywood is occasionally used by some agencies as the blank material for the reflective sheeting face in areas of frequent van­dalism due to gunshots. Wooden sign blanks deform less from gunshots, are easier to repair, and are not as attractive a target as aluminum sign blanks. Plywood, however, does not weather as well as aluminum and, if the edges are not sealed correctly, has a relatively short life. More important, the plywood is heavier than aluminum, thus requiring a stronger post system and increasing the probability of intrusion into the passenger compartment upon impact. Composites such as fiberglass have also been used as sign blank materials with limited success. Early problems with composites included separation of the material and problems with the reflective sheeting adhering to the sign blank. A relatively new sign blank manufactured from recycled thermo­plastic soft drink bottles is available from Composite Technologies [20]. These sign panels are molded with sealed edges, will not bend like aluminum, offer excellent bonding to adhesive sheeting, are weather and corrosion resistant, and are cost-effective compared with current aluminum pricing.

Sign Posts. The sign support must be strong enough to resist the wind and other loads yet safely give way when struck by a vehicle [21]. The loading conditions for which the support must be designed are illustrated in Fig. 7.4. The required size of a signpost is dependent upon the surface area of the sign it is supporting and the prevailing environ­mental conditions. Each state has a series of tables and/or graphs that specify support post requirements based on prevailing wind and ice loads, sign size, and the height of the sign from the ground. These tables provide the information on the support size, embedment depth, and the support type that is required to withstand the environmental loads. The ability of the sign support to operate safely upon impact is dependent on the sign location, features of the surrounding terrain, and the intended method by which the support will give way. All give-way sign support systems operate by (1) complete or partial fracture of the support post, (2) failure of intentionally weakened (frangible) bolts or splices, and

(3) mechanical release methods. These designs allow the support system to either bend at the base (base-bending) or break away into one or more pieces. Sign support systems that do not give way upon impact are fixed-base supports which must be shielded with an appropriate barrier when placed within the traversable area.

Base-Bending Support Types. A base-bending support (Fig. 7.5) is designed to bend over, lie down, and pass beneath the impacting vehicle. How effectively it performs is dependent upon the type of support and the velocity of impact. These supports tend to perform better at lower-speed impacts, which provide sufficient time for them to func­tion as designed. Impacts at high speeds will frequently result in the support’s partially fracturing or being pulled out of the ground. The performance of base-bending supports is more difficult to predict than that of other support types. Their behavior upon impact is influenced by variations in the depth of embedment, the soil resistance, stiffness of the sign support, mounting height of the sign, and the method of effecting the yielding action. One-piece assemblies are typically either driven directly into the ground or set in drilled holes and backfilled. Instead of a one-piece support, the yielding action is often effected by constructing an anchor system and connecting the sign support to the

anchor assembly. The connection can be by direct splicing or the use of commercially available couplers that are designed to bend (fracturing) or break partially (frangible). The advantage of the two-piece assembly is that the anchor system is often not dam­aged during impact, thereby reducing replacement time. Base-bending supports provide a relatively inexpensive support system that reduces the probability that the sign assem­bly will become a deadly projectile to other traffic, pedestrians, and bicyclists.

Breakaway Support Types. Breakaway sign-support systems (Fig. 7.6) are designed to have the system separate, at or near ground level, into more than one piece upon impact. This is accomplished by complete fracture of the support or by the separation of weak­ened splice parts. Wood is the most common material used for complete fracture designs. Weakened splice parts can be field-assembled splices, commercially available splices, or frangible couplings. Frangible couplings are necked down to provide a reduced cross­section. Frangible couplings can be used for single sign supports but are generally used for

large, multiple-support systems. Breakaway support systems typically work best for high­speed impacts where the vehicle has sufficient energy to both break the support and propel it away or over the vehicle.

Mechanical Release Support Types. Mechanical support types include slip base designs (Fig. 7.7), which have flat plates welded to both the sign support and the anchor piece. Upon impact, the plates slide against each other allowing the connecting bolts to release.

Traffic signs are a primary source of information to motorists. The majority of traffic signs consist of sign panels held in place by a single support. Single supports can usu­ally be used for signs as large as 18 ft2 (1.7 m2) in area. The only purpose of the sign support is to hold the sign at the proper position for driver visibility. This requires that this support be strong enough to maintain the sign panel in its intended position while subjected to wind, ice, and snow loads. The magnitude of these forces increases as the sign panel becomes larger in size, until the panel is so large that multiple supports are required. Single sign supports are made of different materials, of various sizes and con­figurations, each capable of withstanding different environmental loads. Considering only the environmental loads and selecting a support system to hold the sign panel at the proper position can result in severe vehicular damage and occupant injury upon impact. Proper sign installation requires that the sign assembly be able to hold its proper position and give way under impact to minimize severity to an errant vehicle and its occupants. This requires the proper design in sign system selection and placement.

Sign supports are classified as single-support and multiple-support systems. Single sign support refers to a support that has no other support, or fixed object, within a 7-ft (2100-mm) radius [18]. Multiple supports refer to installations that are spaced less than 7 ft (2100 mm) from each other, or from other fixed objects. With the closer spacing, it is possible for a vehicle, leaving the roadway at an angle, to impact more than one fixed object or support at a time. Support systems that provide acceptable performance when struck alone can result in severe occupant injury when struck simultaneously with another support. The discussion of this article pertains to single sign supports (i. e., supports installed no closer than a 7-ft (2100-mm) radius to other sign supports or fixed objects). The single mount support types that are used by most agencies include U-channel, wood, square steel tube, and steel pipe. Descriptions of other single mount post types, such as aluminum and fiberglass, are provided at the following FHWA sign support website: http://safety. fhwa. dot. gov/roadway_dept/ road_hardware/signsupports. htm

Breakaway supports are designed and evaluated to operate safely on the basis of the char­acteristics of the vehicle fleet. One of the primary characteristics included in discussions of the impacting vehicle is its weight. While weight is very important, the bumper height is equally important, since it establishes where the vehicle weight is first concentrated on the breakaway support. The majority of the safety evaluation tests are conducted on level terrain. This implies that the impacting design vehicles are striking the breakaway supports at a known height—typically, about 20 in (500 mm) above the ground. Roadside safety could, therefore, be enhanced if wide, level areas are provided along the roadside.

Providing this level roadside is not practical or possible in the majority of roadside situations. Side slopes, ditches, cross-slopes, curbs, and other drainage and terrain

features are necessary roadside design features. How these features can interact with and influence vehicle trajectory and device performance must be considered prior to device installation.

Breakaway support devices are designed to function properly when the slip base is subjected to shear forces. If the point of impact is at a significantly higher point than the design height of 20 in (500 mm), then sufficient shearing forces may not be trans­mitted to the base. The result can be binding of the mechanism and nonactivation of the breakaway device. It is critical, therefore, that breakaway supports not be located near abrupt changes in elevation, superelevation transitions, changes in slope, or curbs that will cause vehicles to become partially airborne at the time of impact. As a general rule, if negative side slopes are limited to 6:1 or flatter between the roadway and the breakaway support, then vehicles will usually strike the support at an acceptable height.

Supports should not be placed in locations where the terrain features can possibly impede their proper operation. Placing supports in drainage ditches can result in erosion and freezing, which can affect the operation of the breakaway support. In addition, vehicles entering the ditch can be inadvertently guided into the support.

Supports should not be installed closer than 7 ft (2100 mm) to other fixed objects. If the supports are placed closer than 7 ft (2100 mm) to other objects that by them­selves are considered acceptable, such as a 3-in-diameter (76-mm) tree, then a vehicle will be able to strike both the support and the object simultaneously. The combined effect of both the tree and the support on the change of velocity can be much higher when impacting both objects simultaneously.

Terrain in the vicinity of the support base must be graded to allow vehicles to pass over portions of the support that remain in the ground or that are rigidly attached to a foundation. Remaining portions of the support that protrude more than 4 in (100 mm) above the ground line over a horizontal span of 5 ft (1.5 m), as presented in Fig. 7.3, can snag the vehicle undercarriage.

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.

Estimates on the number of signs present on our roadways vary drastically. An NCHRP synthesis indicated 58 million signs, while a study for the FHWA estimated that there are approximately 250 million sign assemblies on the U. S. roadway system [15, 16]. Signs contribute an important role in increasing the safety of the roadway by providing regulatory, warning, control, and guidance information to the driver. Every sign that is installed on its own support system, however, provides a fixed object for a potential collision. Even a relatively small sign on an apparent weak support can have severe consequences when struck at high speed.

MUTCD provides information on when traffic signs should be installed. In the case of regulatory signs, and in most cases for warning signs, there are specific warrants that should be met prior to installation [2]. Installing unnecessary signs increases operating and maintenance costs, increases the potential of fixed-object collisions, and reduces sign credibility to the motorist.

The need for traffic signs, roadway illumination, utility service, and postal delivery results in roadside features frequently placed within the roadway right-of-way. (Also see Chap. 6, Safety Systems.) The presence and location of these obstacles varies by roadway type and location. Rural freeways, for example, can be designed where traffic signs are the only obstacles that are added to the roadside. Signs, light pole standards, utility poles, and mailboxes are all frequently encountered on rural collectors. These obstacles, when present, perform a necessary function, but are also potential fixed objects for an errant vehicle. To reduce accident severity it is important that signs, roadway illumination supports, utility poles, and mailboxes be properly designed, located, and placed within the right-of-way. As a general rule, there are a number of options that can be used by design engineers to provide a safe design. In order of pref­erence these options are

• Do not install the obstacle.

• Install it on existing overhead structures, where it does not become an additional fixed object hazard.

• Locate the feature away from the traveled way or behind existing barriers where it will be less likely to be struck.

• Reduce impact severity by using appropriate breakaway or yielding design.

• Shield the feature with a properly designed longitudinal barrier or crash cushion if it cannot be eliminated, relocated, or redesigned.

• Delineate an existing feature if other measures are not practical. Putting up hazard markers is a cost-effective method for alerting motorists to an existing hazard. Obviously, delineators will not make any difference if a driver hits the object, but they might help a driver avoid running off the road at that spot.

Yielding or breakaway supports should be used on all types of sign, luminaire, and mailbox supports that are located within the clear zone. The clear zone is the total roadside area, starting at the edge of the traveled way, that is available for safe use by a vehicle. The desirable width of the clear zone is dependent upon traffic volume, speed, and the roadside geometry. The traversable area is the roadside border area that permits a motorist to maintain vehicle control including being able to slow and stop safely. The traversable area can exceed the desirable clear zone called for in the Roadway Design Guide [10]. Only yielding or breakaway supports should be permit­ted in the traversable roadside, even if it is located beyond the clear zone. In those instances where yielding or breakaway supports are not possible, such as large can­tilever sign installations, shielding with crash cushions or guardrail should be used.

Yielding supports refer to those supports that are designed to remain in one piece and bend at the base upon vehicle impact. The anchor portion remains in the ground and the upper assembly passes under the vehicle. The term breakaway support refers to support systems that are designed to break into two parts upon vehicle impact. The release mechanism for a breakaway support can be a slip plane, plastic hinges, fracture elements, or a combination of these.

The technology of yielding and breakaway support systems has experienced dramat­ic improvements. These improvements were prompted by an increased emphasis on roadside safety and by the large reduction that has occurred in the weights of automo­biles. Many foreign and domestic automobiles on our roadways weigh less than 2250 lb (1020 kg), which was at the bottom of the domestic weight range in 1975. By 1983 the trend to more fuel-efficient automobiles had resulted in approximately 40 percent of auto sales being vehicles weighing less than 2250 lb (1020 kg). Automobiles of 1600 lb (725 kg) and less are now operating on U. S. highways. The typical family automobile weighs somewhere between 2000 and 4000 lb (900 and 1800 kg), with only the luxury and a few other types weighing more. A survey of high-level automotive industry lead­ers, conducted by the University of Michigan, indicates that the total vehicle weight will remain fairly constant [11].

The evolving safety feature environment and the change to the vehicle fleet weights have resulted in a number of revised standard specifications for the testing and acceptance of yielding and breakaway support systems. Requirements for yielding and breakaway support systems were introduced by AASHTO in 1975 and revised in 1985 to keep abreast of new research and development. Two of the most significant changes in the 1975 and 1985 specifications are the reduction in weight of the design vehicle from 2250 lb (1020 kg) to 1800 lb (820 kg) and the change from measures of momentum to measures of change in velocity. These changes, however, do not imply that safety features that satisfied the old specifications do not satisfy revised specifica­tions. For example, the 1985 standard testing guidelines require that supports should impart a preferred vehicle change in velocity of 10 ft/s (3.1 m/s) or less, but not more than 15.4 ft/s (5 m/s). A support that would cause a 2250-lb (1020-kg) vehicle (i. e., 1975 design vehicle weight) to experience an 11-ft/s (3.4-m/s) change in vehicle velocity at a test speed of 20 mi/h (32 km/h) would likely result in 15.4-ft/s (5-m/s) change in velocity when tested under the same conditions with an 1800-lb (820-kg) vehicle (i. e., 1985 design vehicle weight) [12]. These values compare favorably with the change in momentum requirements cited in the 1975 specifications. Supports that had accep­tance test numbers near the preferred values for the old specification can, therefore, be expected to meet the new specification requirement.

Some of the changes in the 1985 AASHTO standard specifications were due to testing guidelines contained in NCHRP Report 230 [13]. NCHRP Report 350 establishes current testing guidelines for vehicular tests to evaluate the impact performance of permanent and temporary highway features, and supersedes those contained in NCHRP Report 230 [13, 14]. These guidelines include a range of test vehicles, impact speeds, impact angles, points of impact on the vehicle, and surrounding terrain features for use in evaluating impact performance. Acceptance testing of yielding and breakaway sup­ports requires evaluation in terms of the degree of hazard to which occupants of the impacting vehicle are exposed, the structural adequacy of the support, the hazard to workers and pedestrians who may be in the path of debris from the impact, and the behavior of the vehicle after impact. The guidelines include requirements for

• The structural adequacy of the device to determine if detached elements, fragments, or other debris from the assembly penetrate, or show potential for penetrating, the passenger compartment or present undue hazard to other traffic.

• A range of preferable and maximum vehicle changes in velocity resulting from impact with the support system. The preferable change in vehicle velocity is 10 ft/s (3.0 m/s) or less. The maximum acceptable change in vehicle velocity is 16 ft/s (5.0 m/s). Note that due to conversion to the SI system the limiting velocity changes were rounded and consequently are not precisely the same as those in NCHRP Report 230 [13].

• The impacting vehicle to remain upright during and after the collision.

• The vehicle trajectory and final stopping position after impact to intrude a mini­mum distance, if at all, into adjacent or opposing lanes.

It is important to use only those support assemblies that have been tested, using the standard specifications, and subsequently approved for use by the FHWA. This is true for city and county jurisdictions where roadway speeds are generally less than what can be expected on state and rural roadways. Impacts with supports can be hazardous even at lower speeds, especially for occupants of a small vehicle. It should be noted that many supports can be more hazardous at low speeds, say 15 to 20 mi/h (25 to 40 km/h), than at high speeds, say 55 to 60 mi/h (90 to 100 km/h). For example, sign supports that fracture or break away can be more hazardous at low speeds, where the energy imparted to the support is not sufficiently large to make the device swing up and over the vehicle. The result can be intrusion of the lower portion of the support into the passenger compartment. Similarly, devices designed to yield are generally more hazardous at high speed, due to the reduced time available for deformation and subsequent passage beneath the vehicle.

The acceptance testing guidelines are intended to enhance experimental precision while maintaining cost within acceptable bounds. The wide range of vehicle speed, impact angle, vehicle type, vehicle condition, and dynamic behavior with which vehi­cles can impact the support cannot be economically replicated in a limited number of standardized tests. The use of an approved device does not, therefore, guarantee that it will function as planned under all impact conditions. However, the failure or adverse performance of a highway safety feature can often be attributed to improper design or construction details. The incorrect orientation of a unidirectional breakaway support, or something as simple as a substandard washer, are major contributors to improper func­tion. It is important for proper device function that the safety feature has been properly selected, assembled, and erected and that the critical materials have the specified design properties.

When possible, and appropriate, the placement of traffic signs, luminaires, and utility and mailbox supports should take advantage of existing guiderail, overhead structures, and other features that will reduce their exposure to traffic. Care should be taken to ensure that supports placed behind existing, otherwise required barriers are outside the maximum design deflection standards of the barrier. This will prevent damage to the sup­port structure and help ensure that the barrier functions properly if impacted. The design deflections are based on crash tests using a 4400-lb (2000-kg) vehicle impacting the barrier at 60 mi/h (100 km/h) and an angle of 25°. The crash tests are conducted under optimum conditions. Other conditions such as wet, frozen, rocky, or sandy soil may result in deflections greater or less than the design values. Typical anticipated deflections are presented in Table 7.1. A summary of FHWA letters of acceptance for sign support types and hardware may be found in the AASHTO Roadside Design Guide [10].

A large number of supplemental advance warning devices have been used by roadway agencies to inform motorists of unusual geometric, operational, or traffic control fea­tures. The use of a device by an agency does not imply that it is a viable or desirable device to use for identified deficiencies. The following concerns should be considered prior to the installation of any device not specified in MUTCD:

• Many warning devices are attempts at political, inexpensive, and/or quick solutions to totally inappropriate roadway conditions. The proper countermeasure for many of these conditions is to correct the fault rather than installing an additional motorist warning. Installing a supplemental warning device should be considered a temporary countermeasure until the inadequate roadway conditions can be corrected.

• MUTCD provides guidance on the proper placement of traffic control devices to provide adequate time for motorists to perceive, identify, decide upon, and perform any necessary maneuver. Section 2C-3 provides guidelines for the minimum placement distances of warning signs, while Sec. 4D.15 specifies the minimum continuous visibility distances that should be present for motorists approaching a traffic signal. The inability to provide the minimum visibility distance is one indication of the need to install an advance warning sign. Guidelines on the height and lateral location of signs are summarized in Fig. 2A-1 in MUTCD [2]. The guidelines of Part 2— Signs of MUTCD should be followed for the installation of all traffic signs.

• Section 2C.03 of MUTCD states that warning signs shall consist of a black legend and border on a yellow background [2].

• Section 2C.02 of MUTCD permits the design of warning signs for special condi­tions [2]. These signs should, however, be constructed with clear and concise verbal messages. Letter legibility and size, combined with placement, must provide a clear meaning and provide ample time for response. Section 1A.10 of MUTCD provides an approval process for new symbols and does not permit the use of symbols that are new or unique and, thereby, not readily understandable by the motorist [2]. The only exception to the provision of nonstandard symbols is where minor modifica­tions to MUTCD symbols are necessary to adequately describe specific design elements of the roadway. An example of a permitted symbol modification is displaying a curve on “Intersection Warning Signs” (W2-2) if the side road occurs in the vicinity of a horizontal curve. Devices that use symbols not contained in MUTCD, or in Standard Highway Signs, are nonstandard devices [2, 9].

• Warning devices should have the same silhouette shape as the device shape. For example a 36-in X 36-in (915-mm X 915-mm) diamond warning sign mounted on a 48-in X 48-in (1220-mm X 1220-mm) square piece of plywood would not satisfy the shape requirement. Dawn and dusk light conditions, fog, and other poor-visibility situations can result in interpreting the warning sign as a guide sign.

• Section 4K.03 of MUTCD permits the use of hazard identification beacons to supple­ment an appropriate warning sign or marker [2]. The hazard identification beacon consists of one or more sections of the circular yellow traffic signal head indication with a visible diameter of not less than 8 in (200 mm). MUTCD prohibits the place­ment of the beacons within the border of the sign except when used with a School Speed Limit sign. If two beacons are used, they should be alternately flashed at a rate of not less than 50 nor more than 60 times per minute.

• Unique situations in the roadway environment can result in the need for changes or additions to MUTCD. Section 1A.10 provides the procedure to be followed for con­sideration of a new device to replace a present standard device, for additional devices to be added to the list of standard devices, or for revisions to recommended applica­tion. Agencies that encounter the frequent need of a unique application are encour­aged to request permission to experiment from the Federal Highway Administration, Office of Transportation Operations (HOTO), 400 Seventh Street S. W., Washington, DC 20590.