The potential traffic safety benefits of lighting are due to an increase in driver comfort and confidence resulting from enhanced vision. This reduces driving stress and tension, increases roadway capacity, and reduces the potential for traffic accidents. The economic return of roadway lighting is greatest in urban and suburban areas with high traffic volumes. Rural locations can also benefit from full, or partial, lighting of decision points such as at isolated intersections, on — or off-ramps, and ramp terminals [3]. For some lighting applications there are warranting criteria that can be used to help determine when lighting should be installed.
Warranting conditions are based on minimum conditions which signify that providing lighting would be beneficial. Satisfying the warrants does not obligate an agency to provide lighting, since warrants are not the only criteria that should be considered. Local conditions such as frequent fog, ice, snow, roadway geometry, ambient lighting, sight distance, and signing could justify modifying the warrants either positively or negatively [3]. Judgments on lighting need should include an assessment of the anticipated benefits, traffic volume, speed, road use during the night, night accident rate, road geometrics, and general night visibility. Some agencies justify lighting based on an economic analysis. This requires placing monetary value on the expected reductions in personal injuries, fatalities, and property damage accidents, in addition to other societal benefits estimated to be realized from illumination.
Warranting conditions have been established for freeways. However, due to the wide diversity of conditions that can exist, there are no established warrants to assist the designer in determining when lighting should be provided for urban streets, highways, walkways, and bikeways. The justification for urban lighting is left to engineering judgment, coupled with perceived user needs and user benefits.
