Category HIGHWAY ENGINEERING HANDBOOK

Breakaway luminaire poles are designed to yield at their base attachment to the foun­dation. There are numerous types of bases currently in service. Some of these are designed for breakaway operation and others are not designed to yield. The nonyielding types have application where vehicle speeds are low and the danger from a falling pole is greater than the hazard of hitting the rigidly mounted pole. A description of the most common base types follows. Not all of these bases are crashworthy.

Direct Burial Base. The direct burial base allows the pole to be directly embedded in the soil. It is the most economical, since it eliminates the need for a foundation. It is the common type of base for wood and is used frequently with concrete and fiberglass reinforced plastic (FRP) poles. FRP poles are the only direct burial poles currently approved for breakaway use. The other types are normally limited to low-speed facili­ties or should be located out of the recoverable area.

Flange Base. Most steel and aluminum poles are fitted with a plate or flange at the base of the pole. With steel poles, this usually involves welding a steel plate to the bottom of the pole. With aluminum poles, a cast-aluminum shoe base is usually fitted to the bottom of the pole. The use of a flange base implies that the flange is to be fas­tened directly to the anchor bolts embedded in the foundation or to some type of breakaway device. When a flange is in direct contact with the concrete, some method needs to be employed that will allow water to flow out and not be trapped in the base of the pole. Trapped water can cause premature failure of the pole due to corrosion on the inside. Flange base designs without breakaway features are not crashworthy and should be restricted to where the hazard from a falling pole is greater than the hazard of impacting the rigidly mounted pole. A flange base is illustrated in Fig. 7.73.

Cast-Aluminum Transformer Base (T-base). T-bases may be steel or cast alu­minum and were originally devised to house the transformer. The T-base (Fig. 7.70) proved unacceptable for storage of the ballast because of moisture and insect damage to the electrical components. However, the cast base proved to have safety advantages, since it yielded and broke apart upon impact. The ballast is rarely stored in the base anymore, but the T-base is still frequently installed because it serves as an electrical junction box and because of its breakaway characteristics.

Frangible Couplings. A number of manufacturers have developed cast and extruded aluminum frangible couplings. The typical coupling (Fig. 7.74) is a short connector attached to the foundation on the bottom and the flange of the pole on the top. Upon impact, the coupling fractures, separating the pole from the foundation. The proper performance of frangible couplings requires proper matching of coupling and pole. Stiffer poles work best with frangible couplings, since the stiffness of the pole results in impact forces remaining in the direction of impact (shear). Flexible poles, such as aluminum poles, bend upon impact, resulting in translation of some of the impact force to vertical forces. This places the couplings in compression and tension, forces the couplings are specifically designed to resist. Frangible couplings often need to be enclosed in skirts to keep dirt and water from entering the conduit and to keep rodents from eating the wire insulation.

Slip Base. Luminaire support slip bases are designed to resist wind and vibration loads while safely releasing upon impact from any direction. A typical base consists of two triangular plates, one welded to the support pole and the other welded to the foun­dation attachment. The plates accommodate three anchor bolts and are slotted to allow release upon impact. If installed correctly, the foundation part of the slip base will be

• Any bolts used to anchor the foundation piece to the foundation must be lower than the plane of the slip base.

• The upper surface of the foundation piece must be no more than 4 in (100 mm) above the surface of the surrounding terrain.

• A keeper plate, 0.05 to 0.03 in (1.3 to 0.76 mm), must be placed between the sur­faces of the slip base to prevent the device from slipping apart in response to wind loads.

• Washers of sufficient strength to prevent deformations into the vee slots must be used between the plates and on the top and bottom.

• The bolts must be torqued to the specified level.

A four-bolt slip base (Fig. 7.75) is also available. Developed by Valmont Industries, it provides added structural resistance to environmental loads and is used extensively by some western states.

Shear Base. The design concept for the shear base is to load the rivets or welds that secure the base to a foundation plate. When struck by a vehicle, the rivets or welds are sequentially sheared and the support breaks away. Typical designs for shear bases are thin-walled stainless steel bases and a family of cast-aluminum bases (not T-bases).

Other. There are other breakaway methods that relate to specific materials used for the pole. These include a fiberglass-reinforced plastic pole with an anchor base that will break above a cast-aluminum base, and several schemes approved for use with aluminum poles.

The foundation for a luminaire pole must provide sufficient resistance to overturning moments caused by the static load of the mast arm plus a wind and/or an ice load. It must be capable of maintaining the correct alignment of the luminaire and able to withstand the impact should the pole be struck. For breakaway poles, the foundation must be rigid enough to allow the breakaway device to operate while not becoming a hazard itself.

Luminaire foundations are perhaps one of the most dangerous constructed hazards on the right-of-way. This is due to their placement or location, structural design, and unsafe wiring systems. Historically, pole foundations have been poured-in-place concrete with steel reinforcing rods and anchor bolts. The requirement that upon breakaway nothing shall project more than 4 in (100 mm) above a chord line drawn between two points 5 ft (1.5 m) apart has caused redesign of concrete foundations. It has been rec­ognized for several years that a problem exists when a foundation is placed on a slope. As early as 1985, a memorandum was issued stating that designers should not allow the slope between the travelway and the foundation to be greater than 6:1. This is even more effective when the diameter of the foundation is as small as possible, thus limit­ing the concrete protruding above the grade line. Eliminating the transformer base allows the foundation to be sized to accommodate the pole bolt circle, which in most roadway size poles is considerably less than the bolt circle for a transformer base. In order to do this, the electrical circuit elements that were formerly housed in the T-base— i. e., splices in the conductors, fuse holders, and surge arresters—must be relocated underground. This requires that the electrical components be capable of being fully submerged and remain watertight. Figure 7.68 provides an example of a small-diameter concrete foundation and adjacent underground electrical junction box design.

LUMINAIRE CABLE & CONNECTOR (ORANGE. MALE)

POWER

CABLE

BREAKAWAY JUNCTION BOX COUPUNG SIMILAR TO TYPE 1 WITH WITH COVERS LIGHT DUTY COVER OR SUP BASE,

зг-mm CONDUIT TO POLE "

NONMETALDC

CONOUIT

GROUNO ROO

DISTRIBUTION BLOCK (RED)

CLEARANCE (TYPICAL FOR ALL REINFORCWG STEEL)

Auger bases are an effective method of reducing diameter of the foundation. Many states use a galvanized steel auger base foundation instead of concrete. Most concrete foundations require 3 in (75 mm) of concrete outside the anchor bolts to provide the necessary strength. Even if the T-base is eliminated, a concrete foundation is 6 in (150 mm) larger in diameter than the pole base it serves. The flat steel top of the auger base foundation can be the same size as the pole base, which minimizes foundation size. Another advantage of the auger base foundation, with the circuit elements under­ground, is the resistance to damage when an accident breaks the pole. When a concrete foundation, with its anchor bolts poured in place, has one bolt damaged, the entire founda­tion should be replaced. The auger base foundation uses relatively short bolts, which are replaceable if damaged. Auger base foundations are easily installed by the electrical crew using the same auger trucks used to drill the hole for the concrete shaft. Electrical crews are not called upon to tie reinforcing steel, set and properly align anchor bolts, or finish the concrete—all tasks that require skill to perform properly. It has been reported that a two – worker crew can install 8 to 10 auger base foundations per day, resulting in significant labor savings. A diagram of the auger base foundation with underground electrical junction box is provided in Fig. 7.69.

Pole foundations cannot always be installed on a 6 to 1 slope, as described in many publications by the FHWA as being necessary to ensure the proper operation of a pole breakaway device. To meet the requirement that no part of the foundation or remaining stub of a breakaway device extend no more than 4 in (100 mm) above a theoretical line connecting possible tire tracks 60 in (1525 mm) apart, it is desirable to reduce the diameter of the foundation. If a concrete foundation is 30 in (760 mm) in diameter, then on a 6 to 1 slope, which is the very best condition expected, there will be 5 in (125 mm) of concrete protruding above the ground plus the anchor bolts and the remaining por­tion of the breakaway device after breaking. Figure 7.70 shows a conventional trans­former base installed on a concrete foundation. A 17-in (430-mm) bolt circle is normal for the current transformer base design and 3 in (75 mm) of concrete is needed outside

LDMINAIRE CABLE & CONNECTOR (ORANGE, MALE)

32-mm CONDUI TO POLE

SHAFT DIAMETER AND LENGTH SHALL BE DETERMINED BY THE SOL TYPE AND THE TOTAL OVERTURN MOMENT OF EACH POLE

HEUX SHALL HAVE A 75 mm PITCH. ALL RADIAL SECTIONS NORMAL TO AXIS (і 3 DEG). / HEUX MUST BE FORMED BY MATCHING METAL DIE,

FIGURE 7.69 Auger base luminaire foundation system with underground modular cable distribution system. Dimensions shown as mm. Conversions: 25 mm = 1 in, 32 mm = 1.25 in, 50 mm = 2 in, 64 mm = 2.5 in, 75 mm = 3 in, 90 mm = 3.5 in, 150 mm = 6 in, 255 mm = 10 in, 305 mm = 12 in.

DIAMETER

FIGURE 7.70 Details of conventional transformer base installed on concrete foundation. (Note: Does not meet latest AASHTO recommendations.)

the bolt circle for strength, resulting in a foundation diameter of 23 in (580 mm). Although there are thousands of existing installations of this type still in service, this design (Fig. 7.70) does not meet the recommendations of the latest AASHTO structural supports specifications for protecting the wiring and the fuses from possible damage when impacted by a vehicle.

The foundation design shown in Fig. 7.71 incorporates the flush-mounted junction box made possible by the submersible wiring system. This allows the bolt circle to be reduced to that required by the pole base plate, usually in the range of 11 to 13 in (280 to 330 mm). The smallest foundation that can be provided is a steel plate that is the size of the pole base. This is attached to a steel shaft that extends into the soil approxi­mately 5 to 8 ft (1.5 to 2.5 m) deep. This foundation design including the submersible wiring is shown on a 3 to 1 slope in Fig. 7.72. These foundations are available from several sources including Dixie Division of Aluma-Form [15] and the A. B. Chance Company [16]. Such a foundation is combined with the flush grade mounted junction box and the submersible Modular Cable System and either a frangible coupling or a slip base pole to achieve an effective breakaway lighting pole installation. Lighting designers cannot always influence the design of a roadway’s shoulders and front slopes, but by using this foundation, designers do all within their power to ensure requirements of the AASHTO Roadside Design Guide are satisfied.

The Modular Cable System mentioned is recommended for use in both breakaway and nonbreakaway pole bases. Because there is only one splice to be made at each pole, with the other connections made by plug-in connectors, the time required to install and the skill level needed of the installer are minimal. Also, when troubleshooting a defec­tive circuit, it is a great advantage to be able to unplug the various circuit elements rather than deal with permanent splices.

MOLDED PLUG & CONNECTOR (ORANGI-

JUNCTION BOX

DISTRIBUTION BLOCK (RED)

One problem that has recently been identified is the potential deadly threat posed by the electric circuits after pole impact by an errant vehicle. There are many documented deaths of motorists who survived the impact with a luminaire pole only to be subsequently killed from the resulting explosion and fire. The explosion and fire are usually caused when the fuel tank ruptures, the vehicle having been caught on an improperly constructed foundation, and the electrical system sparks repeatedly until the fuel explodes. In other incidences, medical personnel have been delayed from attending victims because of the risk of electrical shock from exposed conductors near or under a vehicle.

Past research efforts have concentrated on evaluating the structural breakaway characteristics of luminaire poles. In addition to the need for the pole itself to have breakaway ability, it is recognized that the underground wiring system should also be capable of properly separating. There are a number of reasons for requiring proper separation of the wiring system. One of these reasons is that the size, and associated tensile strength, of the wire cable is sufficient to significantly increase the deceleration rate of impacting vehicles and to also change the trajectory of the falling pole. Another reason is that improper separation of the electrical cabling can result in bare conductors that are still energized, posing an electrical and a possible fire hazard at the accident scene.

Early efforts to reduce electrical hazard concentrated on providing line fuses placed in a breakaway device. However, these widely used “breakaway fuse holders,” which for years have been the standard, have not been certified by testing. Prior expe­rience indicates that they frequently perform improperly during an accident situation. Rather than properly separating, the device frequently pulls off the wire, leaving an exposed end that is potentially deadly. Part of the problem with the breakaway fuse holder is the location of the device in the pole or T-base and the 24 to 36 in (610 to 910 mm) of distribution cable inside the base. This extra length of wire is placed in the pole to allow service crews the ability to pull the wire out of the pole and make the connections to the luminaires. Upon impact this extra length of wire obstructs proper separation of the breakaway fuse, and allows the wiring insulation to be damaged by the fractured pole. The resulting bare electrical conductor poses a safety hazard because of the relatively large voltages used in underground roadway illumination systems.

Most luminaire underground wiring systems operate on 480 V. The reason for using 480 V is that the voltage drop in the copper conductors that supply a given load is only one-fourth the value of the voltage drop when using 120 V and one-half that of 240 V. In addition, luminaires are designed to perform within a certain percent of the rated voltage. Thus for a given percent, such as 10 percent, the allowable drop would be 4 times greater for a 480-V circuit than for a 120-V circuit (48 versus 12 V) or twice that of a 240-V circuit. These factors are additive, so a 480-V circuit requires a much smaller copper wire to deliver the necessary amount of energy over a long dis­tance. Using 480 V is desirable, but proper precautions and installation techniques must be used to reduce the inherent hazard on the public right-of-way.

A modular cable system initially developed by MG2 Inc. and Duraline Inc. elimi­nates a number of problems presented by the current wiring method [14]. This cable system is a submersible, modular plug and cable system that allows the circuit compo­nents (i. e., the low-amperage, fast-acting, current-limiting fuses; the surge arrester where desired; and the conductor splices) to be placed in an underground junction box adjacent to the pole foundation. The circuit breakaway connector can be positively positioned at the top edge of the conduit inside the pole base. Since the stiff, typically no. 4 or no. 6 copper, conducting cables never enter the pole, the system unplugs at ground level. The impact that knocks down the pole will not put stress on the electri­cal cables and will not weaken splices in adjacent poles. Most important, with the modular cable assembly, there is no exposed electrical hazard upon knockdown as can exist with the conventional wiring method. When this system (Fig. 7.67) is combined with a properly installed foundation, the possibility of fire and explosion or electrical shock is significantly reduced if not eliminated. Recent developments have shown that the splices, the surge arresters, the fuse holders, and the ground rod must be placed underground in a junction box adjacent to the pole base to provide the greatest possible degree of safety. This requires that all components be submersible. This design will positively place a breakaway connector in the wiring system at the top edge of the foundation; the fuses are underground, where no damage can occur on the supply side. The Modular Cable System developed by MG2/Duraline was the first of these sub­mersible wiring systems on the market and has proven to be very reliable [14]. By using fast-acting, current-limiting fuses installed below ground, the potential for elec­trical shock and fuel explosions is greatly reduced if not eliminated.

The AASHTO Standard Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, 4th ed. (2001) has included some positive statements strongly encouraging the use of this submersible-type wiring system for all breakaway poles.

Poles are available in a number of materials. The advantages and disadvantages of each follow.

Steel. Steel poles are available in galvanized, painted, powder-coated, and weathering types, plus a combination of powder coating over galvanizing. Galvanized is the most popular of the steel types because of the comparatively low cost and extended life. Painted poles are used primarily when a color is desired, but they require continual maintenance. The powder coating over galvanizing serves the same purpose and requires little maintenance. Weathering steel poles offer enhanced aesthetics but pro­visions must be made for the rusty runoff.

Aluminum. Aluminum poles are popular because of their resistance to corrosion and the resultant low maintenance cost. They have an added advantage of being lighter in weight than most other types. Aluminum poles operate well as breakaway designs when impacted at the design height. Since they are less rigid than steel posts, however, aluminum poles can result in an increased probability of improper breakaway operation when impacted higher than the design height. Aluminum poles are also considerably more expensive than most other types.

Stainless Steel. Stainless steel poles are corrosion-resistant and relatively light­weight. Their high rigidity results in dependable breakaway operation upon impact. They are, however, considerably more expensive than the other pole types.

Fiberglass. Fiberglass-reinforced plastic (FRP) poles are approved for breakaway use both in the anchor base and in the direct burial series. Shaft lengths are currently limited to 47 ft (14.3 m), which means 39 ft (12 m) height for the direct burial series and the full 47 ft (14.3 m) height for the anchor base series. Advantages of FRP poles include no rust, no corrosion, no rot, lightweight, no additional breakaway device required, no maintenance, no electrical shock, and, for the direct burial series, no need for concrete foundation. FRP poles come in many decorative styles and several standard colors.

Wood. Wood is perhaps the least expensive of pole types, particularly in areas where trees are plentiful. They can be treated to resist deterioration from the environment and damage due to insects. The use of existing utility poles for luminaire placement has the advantage of reducing the number of poles on the roadside. The huge mass of wood poles, however, makes it difficult to design them as breakaway, and thus, wooden poles should not be installed on high-speed facilities.

Concrete. Concrete poles are popular in regions where cement and concrete aggre­gates are plentiful. One advantage to concrete poles is that they can be economical. Concrete poles cannot, however, be designed effectively to safely break away upon impact. They are extremely heavy even when made by prestressing concrete. Impacts with concrete poles result in extensive damage to vehicles and severe injury to occu­pants. Prestressed concrete poles, therefore, should not be used within the traversable area, unless shielded, on facilities with design speeds over 30 mi/h (50 km/h). Concrete posts can be a functional and economical type of support on local urban streets if proper consideration is given to placement.

The location of a lighting pole is partially dictated by the lighting scheme selected by the designer for a section of roadway. Using the conventional (cobra head) type luminaires requires the pole to be close to the travelway and therefore, unless it is behind a barrier, most likely to be struck by an errant vehicle. A median barrier-mounted pole is less likely to be struck, but occasionally an out-of-control vehicle will get high enough on the barrier to impact the pole. When this occurs, the danger to oncoming traffic will be increased if the pole is of a breakaway design. Because of this possibility, median-mounted poles are normally not designed as a breakaway type. The lighting scheme that incorporates offset and/or high mast luminaires is the least likely to create a hazard on the roadside, since the poles can be located 40 to 50 ft (12 to 15 m), or farther, from the travelway. In addition to reduced accident rates, this type of lighting reduces maintenance costs due to pole knock­downs.

Pole locations are influenced by the location of sign structures, overpasses, guardrail, roadway curvature, gore clearances, overhead power lines, drainage pipes, drainage structures, underground utilities, and the shoulder slopes, in addition to the luminaire capabilities. The lighting designer must evaluate the eventual consequences of safety, aesthetics, maintenance, and economics when selecting the pole locations. Safety considerations for lighting pole locations include

• Poles should be placed outside the clear zone whenever practical.

• Pole locations should consider the hazards in servicing the lighting equipment.

• Poles should be located to provide adequate safety clearance in the gore areas of exit and entrance ramps.

• Poles should be placed to minimize interference with motorists’ view of the sign, and the luminaire brightness should not seriously detract from sign legibility at night.

• Poles should not be placed where overhead signs will cast distracting shadows on the roadway surface at night.

• Poles on the inside radius of superelevated roadways should have sufficient clearance to avoid being struck by trucks.

• Poles should never be placed on the traffic side of guardrail or any natural or manu­factured deflecting barrier.

• Where poles are located in exposed areas, they should have an approved breakaway feature or device.

• Poles along the freeway should be located at least 4.6 m and preferably 6.1 m or more from the edge of the travelway and include a breakaway device unless located behind a barrier or guiderail or otherwise protected.

• Poles behind flexible or yielding type rails or barriers should provide the necessary clear distance for rail or barrier deflection. The design deflection distance of the particular barrier being used should be checked to ensure that vehicles impacting the barrier will not continue into the lighting support.

• Installing poles on the median, instead of the roadside, should be considered where median width is sufficient (on landscaped medians) and on top of properly designed concrete safety shapes present on narrow medians. Among the advantages with median-mounted poles are that one-half the number of poles are required, the quan­tities of conduit and cable are reduced, house sidelight is minimized, and visibility on the high-speed lanes is improved.

Clear zone is not a constant distance but varies on the basis of the design ADT, the design speed, and the slope, either positive or negative, of the shoulder. Clear zone dimensions are given in the AASHTO Roadside Design Guide [13]. (See Chap. 6.)

The primary purpose of roadway illumination is to increase safety by enhancing night­time visibility. The net safety benefit from increased visibility is influenced by the hazard posed by the roadway lighting or luminaire support acting as a fixed object. If roadway illumination is not warranted, or if it is installed wrong, there is a strong pos­sibility that traffic hazards will be increased rather than reduced by providing illumi­nation. The AASHTO publication Roadside Design Guide requires the lighting designer not only to produce an effective, efficient lighting system but also to consider removing the hazards inherent in such a system [13]. The Roadway Design Guide stresses that safety should be enhanced by considering the following, in decreased order of desirability:

• Remove the hazard from the right-of-way

• Locate the hazard in a place less likely to be struck

• Provide a breakaway support

• Provide a barricade

The most common approach to meeting the safety requirement has been to provide a breakaway structure for the light poles. There are a number of devices that have been tested and approved by the Federal Highway Administration for this purpose, including

transformer bases, frangible couplings, slip bases, and various schemes applicable to a particular type of pole such as fiberglass and aluminum. All these devices will perform as prescribed, but it is up to the designer to use the proper device in the particular situation encountered for the project. The FHWA approval process evaluates only the structural breakaway performance of a tested device, not the structural strength or the possible elec­trical hazard introduced when a pole is struck. The lighting designer must become familiar with the structural load limitations of each tested device in order to match the weight, height, and wind loading demands of the luminaires with the strength of the device being considered. The designer should also consider methods to mitigate or elimi­nate the possibility that damaged electrical wires will be exposed after a pole is knocked down. In urban areas or other locations where pedestrians or cyclists may be in the area where a breakaway pole would fall if struck, breakaway supports are not recommended.

The value of high mast lighting has been highly controversial since its introduction in the early 1960s. Proponents suggested that high mast lighting offered considerable enhancements to visibility. Opponents, on the other hand, argued that high mast lighting was expensive to build, offered little improvement to visibility, and often resulted in light trespass and light pollution. By the early 1980s, new data became available which suggested the superiority of high mast light over conventional systems. The reasons cited were

• An improved visual field negating the “tunnel effect” caused by a limited lateral dimension when using conventional mounting heights. The tunnel effect prevents the driver’s eyes from reaching a reasonable level of retinal stability—a failure believed by some to be the cause of a substantial number of accidents at night [12].

• Improved luminance uniformity within the principal visual field. The uniformity eliminates the need for the eye to adapt to a wide range of luminances, which adversely affects visual acuity. Many experienced engineers are of the opinion that luminance can be reduced when using high mast lighting because of the compensating factors of improved uniformity and reduced veiling glare.

• Disability veiling brightness negatively affects a driver’s visual performance. In practice, luminaires spaced at long distances require large light sources with high beam intensity in the upper angles of the vertical plane. Light emitted from a lumi­naire above 75° can be considered a contributor to glare. High mast luminaires con­fine their distribution within the limits of 60° to 65° and practically eliminate the disability veiling brightness. Brightness from high mast systems is also reduced through geometric arrangement. Increased mounting height and greater offset remove the luminaires from the driver’s active viewing area [12].

• The location of the high mast pole contributes to a clear roadside and results in a reduction in the number of vehicular collisions with fixed objects [12].

• Studies done using the older mercury vapor light sources indicated that on both dia­mond and cloverleaf type interchanges, high mast lighting systems utilized fewer luminaires and less energy than conventional lighting [12].

• There is a growing emphasis across the nation to eliminate or control “obtrusive light.” Keeping this in mind, a designer might want to consider using some of the more newly designed, full-cutoff, high mast luminaires that have been found acceptable and to be aware of keeping the mounting heights as low as feasible for a given situation.

The growth in vehicular traffic combined with the continuous search by transportation authorities for safer and more cost-effective roadway design has resulted in a shift toward multilane roadways. The new freeway designs cannot be effectively illuminated by conven­tional methods using low mounting heights and light sources of limited lumen output. Because of these requirements, high mast systems offer a distinct advantage over alternative systems. High mast systems also offer advantages in cases where future road widening is expected. The poles can be located 50 ft (15 m) or more from the traffic lanes, enabling future road widening without the need for changes in the lighting system [12]. Figure 7.66 shows a typical lighting design.

Conventional roadway lighting has been the cobra head luminaire mounted on a support arm and positioned at the edge of the roadway or, in some cases, out over the roadway. The base of the pole when a breakaway device is present should be a minimum of 15 ft (4570 mm) from the travelway, but 20 ft (6100 mm) is preferred on roadway sections with­out a curb. The travelway is defined as being a continuous traffic lane and does not include an acceleration or deceleration lane merging with a through lane. When a curb is present, the pole with its breakaway device is preferred to be 10 ft (3050 mm) from the face of the curb. If this is not possible, the pole should be closer than 2 ft (610 mm). This will ensure that an impacting vehicle strikes the pole at the designed impact height for proper break­away operation. Breakaway devices should not be used on any pole located where pedestrians are likely to be present, because of the danger to them if the pole falls.

Cobra head luminaires are available in a wide range of full cutoff, semicutoff, and noncutoff beam patterns. All cobra head luminaires have a horizontal lamp position that causes them to produce a large amount of light directly under the luminaire. This requires the designer to closely inspect the calculated average to minimum light level ratios to ensure compliance with values given in the illuminance tables.

Other luminaires that can be used in the same locations as cobra heads utilize a vertical lamp position. These produce a more uniform light pattern, since a smaller portion of their lumens are directed straight down, thus providing a more uniform light level. These luminaires are not available in cutoff types. The two major manufac­turers of this vertical lamp luminaire are Holophane and McGraw-Edison.

High mast luminaires are designed to be mounted on the lowering ring of a high mast pole. High mast luminaires are produced primarily in 400- and 1000-W sizes in a wide variety of beam patterns. New lamps are being developed that have lumen to watt ratios equal to or better than the 1000-W that do not demonstrate the same fragile tendencies [11]. Manufacturers use different designations for their own luminaires, but generally type 2 and type 5 beam patterns are most popular. The beam patterns are also referred to as long and narrow, and symmetrical and nonsymmetrical. Cutoff and

FIGURE 7.61 Evaluation form for non-controlled-access facility lighting.

noncutoff types are used, although not all beam patterns are made in each category. The designer must be concerned with light trespass when using high mast luminaires and should locate them so as not to interfere with adjacent property usage. A tech­nique used by some designers, when high mast poles cannot be located in the middle of the area to be lighted, is to specify an offset-type luminaire mounted on the high

mast lowering ring in lieu of the “traditional” high mast luminaire. This produces a more directional pattern that can reduce the amount of off-premise light.

In addition to the cutoff type fixture, some manufacturers have coined the term “Dark Skies” for fixtures that have a photometric design that not only limits the hori­zontal beam spread, but also limits or completely eliminates any component of uplight from the fixture. Property owners are very concerned about light trespass where com­mercial developments are in close proximity to residential areas. Unwanted light can affect the value of a parcel of property for certain types of usage.

The use of light emitting diodes (LED’s) has created a new type of light fixture. These units are still in the early stages of development but a limited number of manu­facturers do market exterior fixtures using this new technology. The installation costs of LED fixtures are comparable to conventional fixtures but the initial cost of the fix­tures exceeds that of conventional high pressure sodium or metal halide fixtures. However, energy costs using the LED fixtures are much lower than conventional fix­tures. The photometrics of LED fixtures can be controlled to prevent both uplight and trespass on adjacent areas. As the production costs decrease and different styles become available, the use of this type of fixture will likely increase.

Avoiding light trespass is very important near airport runways. Glare from improp­erly placed fixtures or the use of fixtures with uplight components can be a distraction to pilots on final approach. The area around an airport also has height restrictions due to FAA requirements. The lighting designer must be diligent in researching these requirements and using lighting standards that do not exceed the limits. One method is to use offset fixtures at lower mounting heights to comply with height limitations. Most manufacturers also provide shields that prevent overspill of the light output. The use of fixtures referenced above as “Dark Skies” will also aid in meeting the design requirements for a particular location or area.

Offset luminaires are manufactured by several companies under names such as Vector, Turnpike, Multimount, and Interstate. These luminaires are specifically designed for roadway use and resemble a floodlight in appearance but not in beam pat­tern. The offset luminaires are intended to be mounted well away from the roadway edge and aimed at an approximate 45° angle. This design was originally conceived in the 1960s, and a test installation along I-55 south of Memphis was very satisfactory. The original design was large and difficult to handle, but perhaps the greatest handicap that prevented widespread use was the resistance among maintenance personnel due to the difficulty in getting to the pole location when servicing was required. From a safety aspect, the offset was, and still is, a very good choice, since it can be located well away from the travelway and the beam pattern allows a wide spacing between the poles. The development of an individual lowering device has increased the number of locations where the offset can be installed. The individual lowering device (ILD) provides each luminaire with its own lowering cable and latch assembly, as distinguished from the high mast lowering device, which has all luminaires mounted on the same ring and lowered together. The cost of the ILD is much less than that of the high mast device when one to eight luminaires are located on the pole. Four ILDs are the maximum number used on a single pole, but one or two per pole is most commonly used.

Segmented reflectors are special-purpose luminaires that have been successfully used on top of concrete median barriers. The top of concrete safety shape barriers can be as wide as 12 in (300 mm). This limits the anchor bolt spacing, for attaching a luminaire pole, at 6 in (150 mm) in order to provide a minimum of 3 in (75 mm) of concrete around each bolt. The resultant anchor bolt spacing places a height restriction on the pole due to the structural needs required to counteract the overturn moments. Two options are to increase the width of the concrete barrier at the luminaire post, as in Fig. 7.65, or to use segmented reflector luminaires, which require less height to pro­vide proper lighting for multiple lanes.

Two of the segmented reflector luminaires when mounted 40 ft (12 m) high can light up to six or eight lanes on each side of the barrier depending on the width of the inside shoulder. This luminaire was originally developed for use in parking decks and uses a vertically mounted lamp with only a small portion of its lumens directed straight down. Exceptional uniformity ratios and the cutoff pattern make these lumi­naires a good choice when veiling luminance (glare) and light trespass are concerns. Other types of luminaires are also used on top of median barriers. The cobra head is often used, and a traditional high mast luminaire has been found to be very effective in this application, although veiling luminance is a potential problem.

Poles mounted on top of median barriers have a number of advantages and disad­vantages. One advantage is cost, since one pole in the middle of the lighted area can replace two roadside luminaires, requiring only one set of circuit conductors and conduits. The disadvantages include problems of traffic and maintenance safety. Placing the poles on top of the concrete median increases the probability of a pole being struck and landing in the opposing traffic lanes, when compared with offset luminaire pole locations. Maintenance crews are required to work with bucket trucks on the inside shoulder, requiring the closure of the inside traffic lane. Experience of over 10 years with median barrier-mounted poles indicates that few poles are actually struck and the majority of strikes that do occur take place late at night when traffic levels are low. The poles that were struck did not become detached from the anchor bolts, since breakaway devices are not used on the barrier rail poles. The use of bucket trucks to service these luminaires is a potential problem, because the knuckle of the boom can extend over an adjacent lane. The solution in at least one metropolitan area is to use the ILD (Fig. 7.60) with the luminaire, eliminating the need for a bucket truck. This facilitates traffic control and requires a smaller-size crew. Some maintenance depart­ments prefer the ILDs and request their installation on barrier lighting projects.

There have been several types of floodlights and sports lights used in roadway applications over the years as lighting designers have attempted to cope with the increasing numbers of lanes and the confining rights-of-way. In most cases, the high level of accuracy required for proper aiming and the need for special glare shields have limited their usefulness.

An analytical approach to determining if roadway lighting is warranted was developed through the National Cooperative Highway Research Program (NCHRP Report 152) using four comprehensive evaluation forms. The four forms relate to non-controlled – access roadways, intersections, freeways, and interchanges, and are presented in Figs. 7.61, 7.62, 7.63, and 7.64, respectively [10]. The forms are used by multiplying the rating of a characteristic by the difference in its unlighted and lighted weight to obtain a quantitative measure of the effect of that characteristic on driver visual infor­mation needs. After all the characteristics are rated, the scores are summed to obtain an overall measure of driver information needs.

There is an established “minimum warranting condition” of a given number of points for each of the four forms. The exact number of points is determined by assum­ing a rating of 3 for each of the characteristics. It should be emphasized that the mini­mum warranting condition is not firm, but merely a starting point. This method is flexible and permits modifications to fit local needs. This procedure also provides a method for administrators or planners to prioritize lighting projects by using objective standards to determine where lighting would be most beneficial.

The use of artificial daytime lighting is warranted when user visibility requirements are not satisfied by the natural sunlight. Overall tunnel visibility varies considerably with such factors as geometry of the tunnel and its approaches, traffic characteristics, roadway and environmental reflective surfaces, the climate and orientation of the tunnel, and visibility objectives. Comprehensive literature is available on the technical aspects of visibility and lighting of tunnels [9]. Information on lighting levels for tunnels requires a detailed analysis of the tunnel approach characteristics. Tunnel lighting requires considerable experience to achieve proper design.

7.19 ROADWAY REST AREAS [9] •

• Parking areas. The recommended average maintained lighting level is 1.0 fc (11 lx) for both automobile and truck parking areas with a uniformity of 3:1 to 4:1 over the entire area. Special areas that should have the higher levels are handicap ramps, sanitary disposal stations, and other features that require detail viewing.

• Activity areas. The major pedestrian activity areas are restrooms, information cen­ters, and walkways to and from the buildings and the parking lot. Minor activity areas include picnic tables, dog walks, and other walk areas. The recommended lighting level for major areas is 1.0 fc (11 lx) with a 3:1 to 4:1 uniformity ratio. Minor activity areas should be lighted to 0.5 fc (5 lx) with a uniformity ratio of 6:1 [3].

Rest areas are often located in remote areas that are not readily accessible by bucket trucks or other special maintenance equipment. This requires that lighting system components be selected that provide maximum protection against vandalism and require minimal maintenance. One device that has been used to allow pole-mounted luminaires to be maintained, and lamps changed, without using a bucket truck is an individual lowering device (ILD) which allows the pole-mounted luminaires to be lowered to ground level, one at a time, for servicing. This is done with a hand-operated winch that is lightweight and easily portable by one person. One such ILD (Fig. 7.60) that has been designed to DOT requirements and used successfully is manufactured by ITS Products Inc., Dothan, Ala.