The article 'On The Road with Light' presented a detailed review of the three methods for roadway lighting calculations. Not discussed was whether or not roadway lighting is warranted at all, and if warranted, to what degree. RP-8 does not address criteria for 'when' roadway lighting is necessary-the standards only apply to continuous lighting. And what warranting criteria are to be used for 'spot illumination', which applies to many if not most local roads? There are fundamental questions on the circumstances under which roadway lighting is warranted, and these questions deserve careful consideration by public officials and lighting engineers:
1. With fierce competition for limited public financial resources, under what criteria does the investment in roadway lighting provide a reasonable return on investment in terms of collision avoidance?
2. If roadway lighting does provide an appropriate return on investment, does that additional level of collision avoidance provided by lit roads apply at all hours, from dusk to dawn, or is most of the benefit realized before midnight, after which traffic volume may be negligible? Would turning streetlights off after midnight significantly contribute to energy conservation without significantly affecting rates of collision?
3. Could passive alternatives such as reflective pavement markings provide a similar degree of public safety, and if so at what cost?
In the 1970s, in response to an energy crisis, CalTrans decided to remove all roadway lighting from limited access freeways, except for those located at the interchange. In the 30 years since these roadway lights were taken down, CalTrans has not seen the need to reinstall roadway lighting on freeways. CalTrans has now commissioned a study, due for completion in 2008, to determine whether roadway lighting is necessary even at interchanges.
On local roads, most lighting is mounted on wood utility poles, and the spacing requirements are based on the weight distribution factors for the wires. Streetlights mounted on utility poles may offer little, if any, public benefit in terms of collision avoidance.
100 years ago car headlights were very weak, compared to today. Back then there were few examples of roadway striping or reflective markers on the roads.
The question for future research will be to determine the public benefits gained from investment in streetlights, and to compare those benefits to other areas of investment like healthcare or public safety. Until those questions are answered, CalTrans example of not using roadway lighting for freeways deserves careful consideration.
Leo Smith
Suffield, CT
