The following criteria are necessary to ensure satisfactory impact performance of

luminaire supports.

• Use only designs that have been approved as crashworthy by the FHWA.

• The FHWA has established upper limits on the support mass and height of luminaire supports. These limits are applicable even when the breakaway characteristics have proven acceptable by crash testing. The maximum acceptable support weight (mass) is 1000 lb (454 kg), and the maximum luminaire support height is 60 ft (18.3 m). These values are increased from the limits of 600 lb (272 kg) and 50 ft (15.2 m) cited a few years ago. Any further increases in these limits should be based on full-scale crash testing and an investigation of vehicle characteristics beyond those recommended in NCHRP Report 350 [13, 19].

• Breakaway devices are designed to operate by being subjected to horizontal forces (device placed in shear). The devices are designed for this to occur when impacted at a typical bumper height of about 20 in (510 mm). Locating luminaire supports where they will be impacted at a different height will result in forces directed parallel to the support and thereby loading the devices in tension and compression. This results in improper operation of the breakaway device and possibly severe impacts and injuries to vehicle occupants. Superelevation, slope rounding, offset side slopes, curves, curbs, vehicle departure angle, and speed can all influence the striking height of a typical bumper. Negative side slopes should be limited to 1:6 between the roadway and the luminaire to help ensure that errant vehicles strike the support at an acceptable height [13].

• Use a wiring system that allows all circuit components to be shielded from impact, preferably underground, and that ensures that all electrical energy potentially available at the pole foundation surface is limited by the current-limiting fuses. Conductors pro­tected only by a circuit breaker should be not be accessible in the pole base.

• The major cost of a luminaire assembly is the pole, foundation, and breakaway devices. Select luminaires for performance and for a design flexibility that allows more selection of pole locations to produce a lighting system with fewer potential hazards.

• As a general rule, a pole will fall in line with the path of an impacting vehicle. The mast arm usually rotates so that it is pointing away from the roadway when resting on the ground. Consideration, however, must be given to the fact that falling poles may endanger pedestrians and may pose a danger to other motorists.

• A maximum 4-in (100-mm) stub height must be maintained to prevent vehicle snagging. Quick-disconnect electrical circuitry should also be used to facilitate the breakaway mechanism, to reduce the hazard of electrical shock from exposed wiring after impact, and to ease repairs.

• Foundations should be properly sized for surrounding soil conditions. Foundations that move through the soil upon impact place the breakaway mechanisms in bending rather than shear, resulting in improper actuation.

• Curbs, regardless of their shape or height, will elevate an impacting vehicle. The rise in height begins approximately 18 in (460 mm) from the curb and can extend as far as 10 ft (3050 mm). When possible, therefore, luminaire supports should be placed 10 ft (3050 mm) from the curb. If this is not possible, then they should be located closer than 3 ft (610 mm) from the curb. Luminaire poles placed between 3 and 10 ft (610 and 3050 mm) behind curbs increase the chances of improper break­away operation.

• If a luminaire support is placed behind a barrier, it may not be necessary to provide a breakaway feature. In general, if the support is within the design deflection dis­tance of the barrier, then either the barrier should be stiffened or a breakaway pole support should be used.

• Some agencies place luminaire assemblies on top of concrete median barriers. High-angle impacts, or impacts by large trucks or buses, can cause a luminaire mounted on top of a barrier to be struck. Breakaway design is not recommended for this type of installation because of the risk that a downed pole might pose to oppos­ing traffic.

• If a luminaire support is to be placed on top of a concrete barrier, then the barrier must be adapted to fit the pole base. Concrete safety-shape types are typically designed with an approximately 6-in-wide (150-mm) top surface. Since luminaire bases are typically 8 to 12 in (200 to 305 mm) in width, it is necessary to either widen the barrier top to 12 in (305 mm), or flare the barrier in the area of the luminaire.

• Design alternatives should be investigated with the goal of reducing the number of luminaires used along a section of roadway. Higher mounting heights can signifi­cantly reduce the total number of supports needed. Tower or high mast lighting can be used to effectively illuminate major interchanges. This method reduces the num­ber of poles and locates the supports much farther from the roadway.

• It should be noted that some agencies are experiencing problems with the failure of aluminum T-bases due to environmental loads. It is believed that this kind of failure, such as shown in Fig. 7.79, is initially caused by minor impacts from mowing units and other maintenance equipment. This causes small cracks at the bottom flange of the T-base that grow under environmental loads. The result is eventual separation of the bottom flange from the T-base as in Fig. 7.80.

Maintenance activities usually fall into one of two categories: demand or routine. Demand-responsive maintenance is response to random occurrences such as luminaire failures—i. e., lamps, fuses, ignitors, ballasts—or pole knockdowns. Routine mainte­nance is scheduled activities such as group lamp replacement or luminaire cleaning that are intended to produce a certain level of performance of the lighting system and eliminate some of the demand maintenance [18].

7.30.2 Maintenance Guidelines

A comprehensive discussion of roadway lighting maintenance is presented in “Design Guide for Roadway Lighting Maintenance,” IESNA DG-4. In addition to the factors affecting maintenance, this guide includes information for establishing a maintenance management system that will be helpful to agencies attempting to upgrade maintenance activities.

Maintenance must be considered from the earliest design stages of a lighting project. Top-quality materials should be specified and then arranged or located to protect the components from the potential hazards of the environment, whether these be rain, mois­ture, ultraviolet degradation, or threat of vehicular impact. After a system is installed and tested for operation and for component integrity, proper maintenance procedures can produce continued high performance of the roadway lighting system. If the lighting system is not properly maintained, the responsible authorities may expose themselves to potential liability—plus increased costs if expendable items are not replaced as they reach the end of their service life, because they can cause other components to fail.

7.30.1 Maintenance Operations

There are many reasons to routinely maintain a lighting system. The first reason is that only through good maintenance can the system continue to perform as designed. No matter how much knowledge and skill goes into the design, and how much care is put into the installation inspections and final system testing, the system will not provide the performance expected of it if regular maintenance is not performed. In addition to the legal liabilities of a substandard lighting system, the condition of the system reflects the civic concern of the responsible agency. A lighting system containing faults such as burned out lamps, dirty luminaires, or knocked-down poles reflects a poor attitude that is very noticeable. Another factor is that the electrical energy costs are more or less constant even though the light on the roadways may be significantly reduced, so the economic efficiency is decreased.

Before any lighting system is accepted as complete, or preferably before the electricity is turned on, several tests should be conducted to ensure the quality of the components:

Insulation tests. The contractor should measure the conductor insulation resistance to ground of each lighting circuit using a 500-V megohm-range type instrument. A record should be made of each phase conductor’s resistance to ground. The circuits should measure a minimum of 250,000 Q resistance to ground before the power is turned on. The test should be arranged to test splices and all components of the circuit. Ground resistance test. Using an instrument designed for the purpose, the con­tractor should measure the resistance of each ground rod. A written record of the value should be signed and given to the inspector. Any ground rod with a resis­tance of 25 Q or less is acceptable. Additional ground rods, up to a maximum of three at each location, should be installed to reach the 25 Q.

High mast lowering test. Each high mast lighting assembly should be tested by completely raising and lowering the luminaire ring once. Further testing of the latching operation for top latch devices is necessary. Each luminaire ring should be unlatched, lowered a minimum of 6 ft (2 m), then raised and relatched a total of five times to demonstrate its acceptability.

Photocontroller test. The control circuit of the lighting system should be demon­strated to show it operates properly in both manual and automatic modes. The

Voltage tests. The supply voltage at the lighting control center should be measured and recorded. With the luminaires energized and at full brightness, the voltage at the last luminaire of the circuit should be measured to ensure no more than a 10 percent voltage drop is present.