Continuous, complete, or partial interchange lighting is considered to be justified in
the following situations:
• In general, lighting is warranted where the local government agency finds sufficient benefit in the forms of convenience, safety, policing, community promotion, public relations, etc., to pay an appreciable percentage of the cost of the installation, maintenance, and operation of the lighting system.
• Where there is continuous freeway lighting, there should be complete interchange lighting.
• Where complete interchange lighting is warranted but not initially fully installed, a partial lighting system that exceeds the normal partial installation in number of lighting units is considered to be justified.
• Lighting of crossroad ramp terminals is warranted regardless of volumes where the design requires the use of channelizing or divisional islands, and/or where there is poor sight distance.
7.16.3 Freeway Lighting Design Values [7, 8]
The following should be satisfied for proper design when using the illuminance method:
• Continuous freeway and complete interchange lighting should be designed to provide an average maintained horizontal illuminance in the range of 0.6 to 0.9 fc (6 to 10 lx) on the traveled roadway.
• The ramps should be lighted to the same level as the main roadway.
• The point of least illuminance on the roadway should not be less than 0.2 horizontal fc (2 lx).
• An average to minimum uniformity ratio of 3:1 or 4:1 is reasonable. The more uniform design is preferred.
• The higher levels of illuminance should be at the gores and intersections.
• Situations such as high ambient brightness near the roadway or closed circuit television surveillance equipment may justify higher levels of illuminance.
The following criteria should be satisfied for proper design when using the luminance
method:
• The average maintained luminance should be in the range of 0.4 to 1.0 cd/m2 (0.12 to 0.29 foot-lambert, or ft-L).
• The ratio of average luminance to minimum luminance should not exceed 3.5 to 1.
• The ratio of maximum luminance to minimum luminance should not exceed 6.0 to 1.
• The ratio of veiling luminance to average luminance should not exceed 0.3 to 1.
Rapid changes in lighting levels which occur, especially when leaving a continuously lighted section of roadway, may be compensated for by using transition lighting or adaptation techniques. Lighting levels as recommended above should be reduced to approximately one-half the recommended level for about 15 s to allow the eye to adapt.
Lighting on bridges and overpasses should be at the same level as the roadway. It may be desirable to provide lighting on long bridges in urban and suburban areas even though the approaches are not lighted, since lighting enhances both the safety and utility of bridges. Where bridges are provided with sidewalks, lighting is warranted to increase pedestrian safety and security.
