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.