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.)
