Parking Facility Lighting
This is the first of two articles on lighting by Paul Box. This piece concerns surface lots, the other, parking structures. It is one of a continuing series on lighting begun last month in Parking Today. The recognized provisions for lighting is currently being revised by a committee of the IES.. Next month, Consultant Don Monahan will discuss lighting trespass. Editor
he current nationally recognized provisions and recommendations for lighting of parking facilities is given in the Illuminating Engineering Society RP-20-98. (1) This supersedes prior practices and is based upon surveys of existing lighting practices and other sources.
While this article primarily deals with surface lots, some fundamentals common to all parking facilities are presented. Parking has two primary purposes; One has to do with avoiding vehicular or pedestrian accidents and the second with enhancement of personal security. Data from studies of vehicular accidents in parking lots have found about 2/3rds to involve a moving vehicle striking a parked vehicle. Less than 1/3rd involved a moving vehicle striking another moving vehicle, about 6% involved striking fixed objects, and only 1% involved striking pedestrians. (2)
A major study of claims in commercial parking facilities found slip or trip-and-fall pedestrian accidents to account for about 75% of the total claims and slightly over 50% of the cost paid. (3) The study found 7 % of other claims to represent personal assault, 9% vehicle damage, and 5% fixed objects, such as gate damage. Trip-and-fall accidents are often related to wheel stops in pedestrian walking paths or chuckholes in the pavement. Slips-and-falls occur on oily or icy patches.
The term security primarily applies to personal security of users rather than property such as auto theft. The IES recommendations specify two lighting levels; a Basic and an Enhanced Security. It is judgment call to whether a given facility requires the Enhanced Security provision. In both categories, vertical illuminance to discern facial characteristics in the area of lowest horizontal illumination is specified, and is most directly related to Personal security issues.
One aspect of quality is related to the uniformity ratio. In roadway lighting, the average-to-minimum ratio is used, because the only way a driver would be able to see the maximum is by opening the car window and looking straight down when the vehicle is directly under a luminaire. In the case of parking facilities, with much lower speed - -especially the pedestrians - - the observer is more sensitive to the max-to-min. Furthermore, use of this ratio greatly simplifies measurement of existing facility lighting as compared to the point-by-point requirement to determine 'average'.
A second quality element is the vertical illuminance, relating to the ability of an observer to discern a potential assailant at a distance.
A third quality is glare, There are two types: disability glare which reduces the ability to see or identify objects; and discomfort glare which produces ocular discomfort but does not reduce the ability to see. Glare can be limited in parking facilities by reasonable shielding of lamps, reflectors or lenses.
A fourth quality element is obtrusive light. Any upward light in a lot represents wasted energy and contributes to undesirable, sky glow. Downward spill light beyond the boundary may be a problem depending upon the abutting land use. If residential, any significant spill is normally unwanted and may be limited by local ordinances.
Lamps typically used in parking lots include Mercury Vapor, High Pressure Sodium, Metal Halide, and some Low Pressure Sodium. LPS has the highest efficacy (lumens Per Waft) of the various types. Unfortunately, it has the poorest Color Rendering Index (CRI), and the long lamp length makes luminaire lateral distribution poor. Clear Mercury Vapor may also have a poor CRI. White Metal Halide ranks excellent either clear or coated. For a detailed comparison of various lamp characteristics, the reader is referenced to the IESNA Recommended Practice.(1)
Parking lots often utilize roadway lighting units with 2, 3 or 4 luminaires on 30 to 40-foot poles. High mast installations (60 to 100 feet high) with multiple luminaires can be effectively used in large lots. Under certain conditions, wall-mounted units are appropriate, such as in rear or side loading areas. Other categories used are post top and architectural.
Generally, both high mast and roadway lighting type units provide appropriate illumination values, while post top units typically give the best vertical illumination and may be best for small lots. Roadway luminaires are available with various light sources and wattage ratings including the important element of distributions. There are five jg&g types involved with the pattern of illumination on the pavement. Types M and TV are appropriate in most parking locations. Longitudinal distribution is available in Short, Medium and Long.
Maintenance and Operation
In any parking facility it is appropriate to maintain the lighting level as close to the initial values as practical and in no case to typically allow lighting to depreciate below the 'design' level. This can best be accomplished by group replacement of lamps at some stated portion of rated life, such as 80%. Periodic washing of luminaires is essential. Needs will vary depending upon atmospheric conditions.
The primary operation aspect concerns turn-on at the point where artificial illumination will be needed to aid the observer. This has been found in roadway lighting to occur 15 minutes after sunset and (if the system burns all night) turn-off at 15 minutes prior to sunrise. While astronomic dial time clocks are often used, cloudy or stormy weather conditions will affect the thne when artificial lighting is needed, and the preferred method of lighting control is photocell, These are usually set at several footcandles and this operation is best performed at the control centers for the lighting system. An alternate method is the use of a separate photocell in each lurninaire housing.
Recommended Parking Lot Illumination (Maintained)
The IFS Recommended Practice calls for a minimum illuminance of 0.2 horizontal footcandles, or 0.5 for enhanced security. These are measured on the surface. The recommended uniformity ratio, maximurn-to-minimum, is 20 to 1 for Basic and 15 to 1 for Enhanced Security. Minimal vertical illuminance at the point of lowest horizontal illumination is 0. 1 footcandles Basic and 0.25 for Enhanced Security; measured 5 feet above the surface.
Note that these values are maintained, and represent conditions where lamp time in service is at its longest with the maintenance system utilized, and luminaire glassware is at its dirtiest. The economic value of utilizing group replacement of lamps and scheduled washings should be obvious. Over-designing a system to compensate for poor maintenance represents a waste of both capital investment and electrical energy.
1. Lighting for Parking Facilities; RP-20-98, Illuminating Engineering Society of North America; 120 Wall St Red, New York, NY 10005; 1998.
2. Box, Paul C.; Parking Lot Accident Characteristics, ITE Journal Institute of Transportation Engineers; December 1981.
3. Monahan, D.R.; Safety Considerations in Parking Facilities, as presented at the International Parking Conference and Exposition and the Institutional & Municipal Parking Congress, Nashville, TN; April 1995.
4. Box, Paul C.; Relationship Between Illumination and Freeway Accidents; Illuminating Engineering, May/June 1971.