Category HIGHWAY ENGINEERING HANDBOOK

The requirement of adequate visibility is essential for safe traffic operations during both day and night operation. Visibility can be separated into at least three classifications when applied to highway driving: perception, recognition, and decision making [5]. Perception involves the condition of our eyes, the quantity and the direction of the available light, size of the object being viewed, contrast of the object against its background, and the time available for viewing the object. Effective roadway lighting can aid in these tasks by pro­viding the quality of light required by the human eye to increase its visual acuity.

The practice of roadway lighting in the United States is governed by tenets published in the ANSI/IESNA RP-8, American National Standard Practice for Roadway Lighting. In all the editions of RP-8 published from its inception in 1928 through 1983, the criteria for roadway lighting design were based on illuminance (horizontal footcandles). Illuminance is a measure of the amount of light that falls upon a roadway surface. In the 1983 version of the document, alternative criteria were used—one in terms of illumi­nance (footcandles or lux) and the other in terms of pavement luminance measured in candelas per square meter (cd/m2). The preferred method was luminance since it more accurately described that which is perceived by the human eye. Further research into visi­bility has led to a new concept and provides alternative design criteria that may be used. This alternative set of criteria is based on the concept of providing an adaptation level on and adjacent to the roadway that aids in recognition of low-contrast objects.

The visibility of a stationary object on the roadway of a fixed size and uniform luminance is a function of the following:

1. The contrast between the luminance of the object and its immediate visual background

2. The general level of adaptation of that portion of the retina of the eye concerned with the object

3. The amount of veiling luminance (disability glare) entering the eye

4. The difference in eye adaptation between successive eye movements (transient adaptation)

5. The size, shape, and color of the object

6. The background complexity and the dynamics of motion

7. Visual capability of the roadway user

Visibility level (VL) is a metric used to combine mathematically the varying effects of the several factors listed above on the visibility of a standard observer. VL for an object at a particular location on the roadway viewed from a specified point and direc­tion is the amount above the visibility threshold as seen by the observer. Visibility level is a ratio and has no units. The VL as commonly used is based on detection of a “small target” that is flat and 7 in (18 cm) on each side. Small-target visibility (STV) is the weighted average VL for an array of targets as calculated by the visibility mode. A full and complete discussion of the STV method is included in Annex F of RP-8-00 published by the Illuminating Engineering Society of North America, New York (www. iesna. org).

PART 2

ROADWAY LIGHTING

C. Paul Watson, P. E.

Formerly, State Electrical Engineer
Alabama Department of Transportation
Montgomery, Alabama

Nelson Russell, P. E.

Manager, Electrical Department
Volkert & Associates
Mobile, Alabama

Brian L. Bowman, Ph. D., P. E.

Professor of Civil Engineering
Auburn University
Auburn, Alabama

Part 2 of this chapter presents considerations in the selection of lighting for freeways and other types of roadways. Both standard and high mast lighting are addressed. Roadside safety and the application of various types of bases are discussed and illustrated. Information on construction, acceptance testing, and maintenance is presented. An exten­sive list of references, which are noted in the text, concludes the section. Portions of this material were derived from studies made under a Federal Highway Administration Project, “Design, Construction and Maintenance of Highway Safety Features and Appurtenances.”

714 BENEFITS AND FUNDAMENTALS OF LIGHTING

Properly designed and installed roadway lighting can result in significant reductions in
nighttime traffic accidents, act as a deterrent to crime, increase commercial activity,

and improve aesthetic value. Roadway lighting increases traffic safety by enhancing the visibility of potential roadway hazards, other vehicles, pedestrians, and roadway geometrics. Pedestrians are among the largest beneficiaries of lighting installed on urban streets. Studies indicate reductions of up to 80 percent in pedestrian accidents and reductions ranging from 20 to 40 percent for all types of night accidents [1]. Another study identified a 40 percent reduction in the ratio of night accidents to day accidents resulting from the installation of roadway lighting on freeways [2]. While these figures are significant, it is anticipated that the safety benefits derived from the installation of roadway lighting will become even more pronounced in the future. This is due to the increasing age of the driving population and the significantly reduced visual abilities of persons over 65 years of age. The savings realized by accident preven­tion alone can often justify the costs of a modern lighting system [3].

Although much progress has been made in improving lighting system efficiency and effectiveness, there are still many streets, particularly in small communities, that are not lighted in accordance with present guidelines. This is primarily due to the scarcity of local funds, which can be mitigated by the use of federal funds on qualifying projects. Roadway lighting has been recognized as a viable countermeasure for increasing traffic safety since 1966, when federal legislation enabled federal aid expenditures for construction and maintenance of roadway lighting [4].

The benefits of providing roadway lighting include enhancing traffic safety, improving pedestrian visibility, deterring crime, improving commercial interests, and promoting com­munity pride. The actual benefits obtained are dependent upon the type of facility and area in which the lighting will be installed. Only the traffic operational and safety benefits obtained from the proper design and installation of roadway lighting are discussed in this chapter. It should be noted that properly designed and installed roadway lighting can result in roadway facilities operating almost as efficiently and safely at night as during the day­time. Lighting cannot, however, be expected to achieve the same safety levels as daytime operation, because of the influence of other factors, such as fatigue, higher speeds, and intoxication, which make a greater contribution to nighttime accident frequency.

A tool such as that shown in Fig. 7.57 can be constructed out of pipe to straighten twisted U-channel posts [48]. Similar devices with a metal U-shape at the end of a pipe handle can be constructed to realign shaped wood posts and square tubing. A large pipe wrench can also be used to realign U-channel and square-tube supports. Small signs should be mounted at 90° to the road.

U. S. Customary Units, in

7.9 REFERENCES ON SIGNING

1. Manual on Uniform Traffic Control Devices, American Association of State Highway Officials and National Conference on Street and Highway Safety, Washington, D. C., 1935.

2. Manual on Uniform Traffic Control Devices for Streets and Highways, Federal Highway Administration, U. S. Department of Transportation, Washington, D. C., 2003.

3. Traffic Control Devices Handbook, Federal Highway Administration, U. S. Department of Transportation, Washington, D. C., 1983.

4. Bowman, Brian L., NCHRP Synthesis of Highway Practice 186: Supplemental Advance Warning Devices, Transportation Research Board, National Research Council, Washington, D. C., 1993.

5. Harwood, Douglas W., NCHRP Synthesis of Highway Practice 191: Use of Rumble Strips to Enhance Safety, Transportation Research Board, National Research Council, Washington, D. C., 1993.

6. Cunard, Richard A., NCHRP Synthesis of Highway Practice 157: Maintenance Management of Street and Highway Signs, Transportation Research Board, National Research Council, Washington, D. C., 1990.

7. “1988 Annual Report on Highway Safety Improvement Programs,” Report of the Secretary of Transportation to the United States Congress, Report No. FHWA-SA-88-0003, Federal Highway Administration, U. S. Department of Transportation, Washington, D. C., 1988.

8. Lewis, Russel M., NCHRP Synthesis of Highway Practice 106: Practical Guidelines for Minimizing Tort Liability, Transportation Research Board, National Research Council, Washington, D. C., 1983.

9. Standard Highway Signs, Including Pavement Markings and Standard Alphabets, 2004 Version, U. S. Department of Transportation. Federal Highway Administration, Washington, D. C., 2004.

10. Roadside Design Guide, American Association of State Highway and Transportation Officials, Washington, D. C., 2002.

11. Delphi V—Forecast and Analysis of the U. S. Automotive Industry through the Year 2000, University of Michigan Transportation Research Institute, Ann Arbor, July 1989.

12. Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals, American Association of State Highway and Transportation Officials, Washington, D. C., 2001, and Interim Specifications, 2002, 2003, and 2006.

13. Mickie, J. D., “Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances,” NCHRP Report 230, Transportation Research Board, Washington, D. C., March 1987.

14. Ross, Hayes, Jr., Dean L. Sicking, Richard A. Zimmer, and Jarvis D. Mickie, “Recommended Procedures for the Safety Performance Evaluation of Highway Features,” NCHRP Report 350, Transportation Research Board, National Research Council, Washington, D. C., 1993.

15. Cunard, Richard A., NCHRP Synthesis of Highway Practice 157; Maintenance Management of Street and Highway Signs, Transportation Research Board, National Research Council, Washington, D. C., 1990.

16. Perkins, David, D., “Manual on Countermeasures for Sign Vandalism,” Report No. FHWA – IP-86-7, Federal Highway Administration, Washington, D. C., September 1986.

17. Texas Transportation Institute, “State of the Practice in Supports for Small Highway Signs,” Technology Sharing Report 80-222, Federal Highway Administration, Washington, D. C., 1980.

18. Heinz, Ronald E., “Request for Technical Assistance: Sign Support Design,” Memorandum, Federal Highway Administration, Highway Design Division, Washington, D. C., July 1985.

19. Hayes, E. Ross, Jr., Jesse L. Buffington, et al., “State of the Practice in Supports for Small Highway Signs,” Federal Highway Administration, Washington, D. C., 1980.

20. Composite Technologies Company, Signs Manufactured from 100% Landfill-Destined Plastic, undated brochure, Composite Technologies Company, Dayton, Ohio.

21. Standard Specifications for Highway Signs, Luminaires and Traffic Signals, American Association of State Highway and Transportation Officials, Technical Committee, Washington, D. C., 1985.

22. Phillips, David K., “Steel Flanged Channel Posts for Small Highway Sign Supports,” techni­cal advisory, Federal Highway Administration, Office of Engineering, Washington, D. C., September 27, 1983.

23. Staron, L. A., “Minute Man Breakaway Device,” letter, Federal Highway Administration, Federal Aid and Design Division, Washington, D. C., January 1987.

24. Heinz, Ronald E., “Splicing of U-Channel Steel Posts,” memorandum, Federal Highway Administration, Highway Design Division, Washington, D. C., October 1984.

25. Noel, Leon M., “Timber Sign Supports,” letter, Federal Highway Administration, Highway Design Division, Washington, D. C., August 1982.

26. Hove, R. W., “Ground Mounted Signs: Timber Sign Supports,” memorandum, Federal Highway Administration, U. S. Department of Transportation, Washington, D. C., May 1984.

27. Allied Tube and Conduit, Qwik-Punch and Qwik-Coat Systems, Harvey, Ill., January 1991.

28. Noble, Glen, “Support System Acceptance,” memorandum, Unistrut Corporation, Wayne, Mich., November 1992.

29. Staron, L. A., “Quick-Punch Sign Supports,” letter, Federal Highway Administration, Federal Aid and Design Division, Washington, D. C., October 3, 1986.

30. Xcessories Squared, Soil Stabilizer (brochure), Auburn, Ill., February 23, 1995.

31. Staron, Lawrence A., Geometric and Roadway Acceptance Letter No. SS-25, Federal Highway Administration, Federal Aid and Design Division, Washington, D. C., June 4,

1991.

32. Staron, L. A., Geometric and Roadside Design Letter No. SS-38, Federal Highway Administration, Federal Aid and Design Division, Washington, D. C., November 1993.

33. Dent Bolt, Trinity Industries, Inc., Highway Safety Products, Dallas, Tex.

34. Staron, L. A., ACTION: Breakaway Sign Supports, memorandum, Federal Highway Administration, Federal Aid and Design Division, Washington, D. C., September 1993.

35. Franklin Steel Eze-Erect™ Sign Posts, Franklin Steel, Franklin, Pa., May 1989.

36. The Minute Man™ U-Channel Breakaway Signpost System, Marion Steel Co., Marion, Ohio,

1992.

37. Staron, L. A., “Splicing of Steel U-Channel Posts on Small Sign Supports,” memorandum, Federal Highway Administration, Federal Aid and Design Division, Washington, D. C., September 1991.

38. Unistrut-Telespar™ Sign Support System, Unistrut Corporation, Wayne, Mich., 1986.

39. Allied Square Tube Signposts, Allied Tube and Conduit, Harvey, Ill., February 1992.

40. 14 Ga. Qwik-Punch System, Allied Tube and Conduit, Harvey, Ill., September 1991.

41. Roadside Improvements for Local Roads and Streets, Office of Highway Safety, Federal Highway Administration, Washington, D. C., October 1986.

42. Staron, L. A., Geometric and Roadside Acceptance Letter No. SS-27, Federal Highway Administration, Federal Aid and Design Division, Washington, D. C., May 1992.

43. Staron, L. A., Geometric and Roadside Acceptance Letter No. LS-23, Federal Highway Administration, Federal Aid and Design Division, Washington, D. C., January 1991.

44. Van Ness, Norman, Acceptance Letter to Southwest Pipe Inc., Highway Design Division, Federal Highway Administration, Washington, D. C., July 1986.

45. Hanna, Howard, “Evaluation of the Upper Hinge Mechanism of Multiple Leg Breakaway Sign Supports,” memorandum, Federal Highway Administration, Program Development Division, Washington, D. C., October 1987.

46. “Maintenance of Small Traffic Signs—A Guide for Street and Highway Maintenance Personnel,” Federal Highway Administration, Report No. FHWA-RT-90-002, Washington, D. C., 1991.

47. Nettleson, Tom, Signs Maintenance Guide, Forest Service, U. S. Department of Agriculture, October 1979.

48. McGee, H. W., et al., “Sign Fabrication, Installation and Maintenance,” Federal Highway Administration, Report No. FHWA-SA-91-033, Washington, D. C., May 1992.

49. Horne, Dwight A., “Slip Base Triple Square Supports in Standard Soil,” Letter No. SS68B, U. S. Department of Transportation, Federal Highway Administration, Washington, D. C., 2004.

It is not necessary to repair each hole in a sign. When a hole does not damage the mes­sage or symbol and does not create the impression of a sloppy sign, then repair may not be needed. The following procedures can be used to make field repairs on signs.

Retroreflective Aluminum Sign Panels

• Remove all damaged background sheeting and legend. Usually this means about 1 in (25 mm) from the edge of the hole. A retractable-blade knife is a useful tool for this.

• Straighten the sign (flatten out the hole puncture nipple area) using a ball peen hammer and a flat surface (truck bed, trailer bed, or a fender dolly).

• Remove any additional sheeting damaged during straightening.

• Clean the entire area with xylol; then apply varnish maker’s and painter’s (VM&P) naphtha.

• Patch the hole or puncture on both sides of the sign backing material using 3M Company No. 425 UAL aluminum foil tape or equal. Use a squeegee to apply firm pressure on both sides of the sign. On large holes, start placing the foil at the bot­tom of the hole, overlapping each strip about f4 in (6 mm) in shingle fashion as you move up, and cover the hole area.

• Apply retroreflective background sheeting, extending it at least L in (13 mm) beyond the foil tape strips.

• Replace damaged legend with die-cut, pressure-sensitive, prespaced letters, borders, or symbols and firmly squeegee them into place.

• Seal edge of new background sheeting and legend with 3M Company No. 700 edge sealer or equal. If the sign is subject to snow burial and replacement sheeting extends to the edge of sign, place 3M Company transparent film (No. 639 or equal) along that top edge.

Instead of making small patches to signs with holes, a portable double-roller unit for applying a full-sized sign face to a sign blank in the field can be used. After patching the holes, remove the paper material protecting the adhesive backing. Carefully align the new sign face sheet with one edge of the sign blank and spread the new sign face over the sign blank as smoothly as possible by hand. Then crank the sign blank with new sign face through the portable roller unit to properly pressure-seat the new sign face. Seal the edge of the new sheeting if necessary.

Retroreflective Plywood Panel Signs

• Remove all loose wood on both sides of the sign and all damaged sheeting.

• Fill holes with wood filler, let the surface set, and sand smooth if you think the holes need to be filled for a field repair. Allow filler to harden. Small holes can be covered by foil tape without filling.

• Wipe areas with clean cloth.

• Cover holes on both sides of the plywood sign blank with 3M Company No. 425 UAL aluminum foil tape or equal. Apply firm pressure to the tape on both sides of the plywood sign back using a squeegee. On large holes, start placing the foil at the bottom of the hole, overlapping each strip about 1/4 in (6 mm) in shingle fashion as you move up and cover the hole area.

• Apply retroreflective background sheeting, extending it at least У2 in (13 mm) beyond the foil tape strips on the face of the sign.

• In the area covered by the patching, replace any damaged legend with die-cut, pressure – sensitive, prespaced letters, borders, or symbols and firmly squeegee them in place.

• Seal edge of new background sheeting and legend with 3M Company No. 700 edge sealer or equal. If the sign is subject to snow burial and replacement sheeting extends to the top edge of the sign, place 3M Company transparent film (No. 639 or equal) along the top edge.

• Lightly spray a sealing film of flat black enamel paint (use an aerosol can) over the aluminum foil tape covering the holes on the back of the sign panel. Be careful to keep paint off the front sign face, because paint will destroy the night retroreflec – tion. If your agency paints plywood sign backs some color other than black, use an appropriate color if possible.

Sign legibility can be restored or improved by general cleaning and removal of foreign substances from the sign face. Sign cleaning products are commercially available for use in removing common soil and severe contaminants such as paint and adhesives. In the majority of cases the required cleaning products can be obtained from local hardware stores. Strong solvents and incomplete removal of the contaminant can damage the sign reflectivity. Strong solvents should be trial-tested prior to application on the sign. The following steps can be used to clean signs of common soil and some contaminants [47]:

• General sign cleaning: Mild, nonabrasive cleaners and detergents suitable for painted or enameled surfaces are recommended for removal of common soil. Cleaners should be free of strong aromatic solvents or alcohols and be chemically neutral (pH of 6 to 8 is recommended).

• Pollen and fungus can be removed by washing the surface with 3 to 5 percent solutions of laundry bleach (sodium hypochlorite). This should be followed with detergent wash and a clear water rinse.

• Lipstick, crayon, tar, oil, bituminous materials, and some oil-based paints can often be removed with mild solvents such as mineral spirits (toluene), kerosene, heptane, or naphtha. Wipe the contaminated area lightly with a soft cloth saturated with the solvent. Continue wiping lightly until the contaminant is removed. If this does not work, then try the next step.

• Wipe the contaminated area with a soft cloth moistened with lacquer thinner. Continue wiping lightly until the contaminant is removed.

• A nighttime reflectivity check or a nighttime visual inspection of all signs from which contaminants have been removed should be conducted.

The decision on the appropriate action for damaged sign panels is a field judgment. Minor bending of a sign will prevent headlights from illuminating the sign at night. Signs with minor bends can be repaired by removing the sign from the post and straightening the sign. Signs that are badly bent cannot be properly repaired in the field. Attempts to straighten badly bent signs result in cracking and peeling of the sign face material. Many agencies consider it more economical to replace rather than repair signs that maintenance workers judge to be badly worn or damaged [46].

There are field repair kits available with pressure-sensitive reflective background sheeting and die-cut pressure-sensitive prespaced letters, borders, and symbols. It is often difficult, however, to properly apply these materials under field conditions. In addition to the difficulty of field repairs, a regulatory or warning sign should be placed on the post while repairs are being made. This sign may as well be a replace­ment sign and a more economical and durable repair made in the controlled environment of a shop operation. Do not take down a sign without immediately positioning a replacement. Extra signs should be placed in the service truck prior to leaving the garage. If field replacement of reflective sheeting is performed, however, the proper procedure provided by the manufacturer must be followed.

Techniques to reduce incidents of sign vandalism include steps that address the reasons

for vandalism, enable the prosecution of offenders, ease maintenance, and make it more

difficult to perform the vandalism. Consider the following to reduce vandalism:

• The theft and damage to many street name signs is due to the similarity to someone’s name. Vandalism to signs can often be reduced by adding St., Ave., or Blvd. to the sign.

• Use only standard signs. Signs that have an unusual message experience a higher vandalism rate.

• Use sign blank materials that are less susceptible to specific types of vandalism. Thicker-gage aluminum sign blanks can be used in areas that are subject to damage by bending. Plywood sign blanks are less susceptible to gunshots. Aluminum signs, when struck by gunshot, are indented over a /f-in-diameter (12.5-mm) circle per bullet hole, resulting in severe chipping and loss of reflectivity and legibility. Plywood signs remain legible even with numerous bullet holes. Plywood signs are also a less attractive target than aluminum signs, since they provide less noise and movement when used for target practice.

• Place an agency identification sticker on the back of each sign. This sticker should have a unique number for each sign, the agency name, whom to contact if the sign is found, and a warning about the legal consequences of stealing or damaging the sign. The identification sticker enables law enforcement officials to prosecute indi­viduals stealing or vandalizing the sign. The date of installation can also be placed on the sticker for maintenance information.

• Apply protective coatings to the sign face to ease the removal of foreign substances. Clear coatings, such as product number 711 or 731 from the 3M Company, can be applied by spraying, roll coating, or hand brushing. Transparent overlay films such as Scotchlite brand graphic overlay (GOF™) from the 3M Company are also available. The clear coat­ings and overlays allow the removal of crayon, paint, lipstick, and other contaminants with the use of strong solvents that would normally harm uncoated sign face material.

• Support twisting or removal can be reduced by installing approved supports of a heavier gauge and using anchor plates. Driven sign supports, as opposed to those installed by drilling and backfilling, are less susceptible to twisting.

• Use commercially available antitheft fasteners that make it difficult for vandals to remove signs. These fasteners include Tufnet, Teenut, aluminum fluted nuts, blind aluminum rivets, and Vandalgard nuts as illustrated in Figs. 7.54 through 7.56.

NEOPRENE

OR

NYLON WASHER

PLYWOOD SIGN

TUFNUTS’

t JUNCTION

NYLON WASHER

TYPICAL INSTALLATION PROCEDURE

STEP 1: INSTALL FIRST TUFNUT (N0.1) FINGER TIGHT AS SHOWN.

STEP 2: INSTALL SECOND TUFNUT (NO. 2) FINGER TIGHT AS SHOWN.

STEP 3: INSTALL WRENCH AT JUNCTION TO TIGHTEN (OR LOOSEN) AS NECESSARY

STEP 4: REMOVE TUFNUT NO. 2; THEN INSTALLATION IS COMPLETE.

SINGLE TUFNUT IS DIFFICULT TO REMOVE BECAUSE OF ITS SHAPE

ALWAYS USE FOUR TUFNUTS FOR EACH SIGN INSTALLATION.

TYPICAL TUFNUT (FOR 10 DIA CARRIAGE BOLT)

TYPICAL TUFNUT

(FOR 10 DIA CARRIAGE BOLT)

REMOVAL

FIGURE 7.55 Vandalgard sign fastener.

PLACE NYLON OR FIBER WASHER

BETWEEN SIGN FACE AND PALLET

WOOD POST SUPPORT (DIMENSION VARIABLE]

CARRIAGE BOLT JUST LONG

ENOUGH TO REACH POINT A

NYLON WASHER

IFOR WOOD POST SUPPORTS»

SPECIAL TOOL NO 2

МШ

ALUMINUM FLUTED NUT5 (FOR ALUMINUM DELINEATORS

AND SIGNS ON U – CHANNEL POSTS)

BLIND ALUMINUM RIVETS (FOR ALUMINUM AND 13

PLYWOOD SIGNS ON

U-CHANNEL POST9)

FIGURE 7.56 Teenut, aluminum fluted nut, and blind aluminum rivet sign fasteners.

7.8 MAINTENANCE OF TRAFFIC SIGNS

Continuing maintenance is required to ensure that traffic signs function for their intended purpose. Proper maintenance of all signs is important since the condition of the signs is a visual statement on the competency of the roadway agency. Regulatory and warning signs that are missing or in poor condition pose safety hazards to
motorists and can result in tort liability. Regulatory and warning signs must be repaired as soon as a defect is noticed. All of the signs on an agency’s roadway system should be inspected periodically to determine that their orientation and retroreflectivity properties are adequate for nighttime visibility.

Damage to traffic signs can occur as the result of environmental and wind load, accidents, improper installation, end of effective service life, and vandalism. Repairs can be required for the sign panel, the sign support, or both.

Sign vandalism costs millions of dollars each year in increased maintenance costs and is a contributing cause to many accidents as well. In addition to the accident itself, van­dalized signs can expose the roadway agency and municipality to tort liability cases. Surveys of state and local agencies indicate that an average of 30 percent of all sign replacement and repair is due to vandalism and that an average of 30 percent of the sign maintenance budget is required for vandalized signs. Acts of sign vandalism are categorized as destruction, mutilation, and theft [16].

7.11.1 Destruction

Destruction occurs when the sign support or sign face is physically damaged to the extent that it no longer serves its intended purpose. Destruction vandalism includes damage from

• Gunshot

• Thrown projectiles such as rocks and bricks

• Sign bending

• Sign or support burning

• Deliberate sign or support knockdown

• Sign cutting with snips or saw

• Support twisting that results in improper orientation

• Support cutting

7.11.2 Mutilation

Sign mutilation occurs when the installation is altered or defaced in such a manner that the sign is illegible or loses its nighttime retroreflectivity characteristics. Examples of sign mutilation include

• Application of paint by spray or brush

• Application of unauthorized stickers or decals

• Contamination by caustic substances

• Alteration of sign legend by crayon, lipstick, or ink markers

• Reorientation of the sign panel

• Scratching the sign surface

• Peeling or removing reflective sheeting

7.11.3 Theft

Theft is the unauthorized removal of a sign assembly or any of its parts. Some common reasons for theft include

• Home decoration

• Relationship of the sign legend to an individual’s name or interests

• Construction or scrap value of the wood, aluminum, or metal parts

• Firewood

• Uniqueness of the sign legend

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.

The sign blank and its mounting hardware become a structural component of the sign assembly upon impact. Slip base and frangible coupler designs of multiple-sign-support systems require the sign panel hardware, and the upright signposts, to provide the

rigidity necessary for proper operation. This includes providing sufficient resistance to activate the hinge and to prevent intrusion of the sign and impacted support into the passenger compartment. Proper hinge activation also requires that no portion of the pri­mary sign, or any supplemental signs, be attached to the support posts below the hinge. In addition, no portion of the sign panel should extend lower than 7 ft (2100 mm) above ground level.

Fastening of sign panels to multisupport sign systems usually requires the use of stiffeners to provide the required rigidity. The exception to this is for relatively small surface area signs, which require multiple supports because of their shape, and for wooden signs. Consult state specifications for installation requirements. Clamps are used to fasten the stiffeners to S – or W-shaped beam posts, eliminating the need to drill into the post itself. Fastening details using aluminum Z-bar as the stiffener were presented in Fig. 7.43, with other common methods presented in Figs. 7.44 and 7.45. U-channel posts can also be used as stiffeners for large signs. When U-channel is used, it should be galvanized and should weigh no more than 2.5 lb/ft (3.7 kg/m). U-channel of 2.0 lb/ft (3 kg/m) is sufficiently strong to withstand wind loads of 70 mi/h (130 km/h).

Figure 7.53 presents the configuration of stiffeners for various sign sizes on a dual multiple-support system. Signs with a height of 24 in (600 mm) or more should be reinforced with two stiffeners placed a distance of one-fourth the sign height from the top and bottom of the sign. The stiffeners should not extend closer than 2 in (25 mm) to the sign edge. Supplementary signs, added to the bottom of the primary sign, should

be attached to the sign stiffeners and not to the posts. Signs should never be allowed to extend above and below the hinge at the post. Extending the sign at the hinge can cause the hinge to bind and improper operation upon impact.