IMPACT PERFORMANCE CRITERIA

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.