ROADSIDE SAFETY

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.