Unit Power Density (UPD) is the ratio of the connected power for a roadway lighting system to the corresponding area of the roadway. UPD directly describes the amount of connected electrical power required by the lighting system, but it also provides information on proportional changes throughout tha lighting system. A 10% increase in UPD between systems using the same wattage lamps corresponds to 10% more of most everything except the wiring in the ground (because the project is still as long as it is): lamps, installed lumens, ignitors, ballasts, photocells, luminaires, poles, arms, brackets, foundations, electrical controls, energy use and cost, maintenance requirements and labor, as well as pollutions associated with equipment production, delivery, and installation and of course energy generation and transmission. The proportion may apply even to installation costs and labor, light pollution and mercury from spent lamps going into landfills. Comparisons between systems with different wattage lamps are less directly proportional, because the number of poles and luminaires may be significantly different (because of the significant difference in "coverage" for each pole). Even so such comparisons across wattages can be informative.
UPD is calculated for one "typical" luminaire cycle as:
Eq. 1: UPD = (# of luminaires) * (watts / luminaire) *1.15 / (area of roadway)
The number of luminaires per luminaire cycle is two for staggered (used throughout this work) and opposite patterns and one for single-sided patterns. Watts are the total input watts for each luminaire, including any ballast losses. The factor of 1.15 is included to account for the anticipated spacing reductions in some areas, compared to the "typical" spacing calculated according to the National Standard Practice for Roadway Lighting, ANSI/IESNA RP-8-00. The area of the roadway is the extent of the traveled way, equal to the length of the luminaire cycle times the number of lanes times the width of each lane. Shoulders are not included in the area of the roadway.
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