FLH PDDM Chapter 9 - Eastern Federal Lands Highway Division

9.4.F. Other Design Elements. (continued)(2) Rural Intersections. Where warranted, illuminate rural intersections to provide visibility of other trafficand physical features that are potential collision objects. Place a minimum of two luminaries, and preferablyfour, per intersection. Maintained average illuminance of the roadway between luminaries shall be 10 to 12lux with a maintained minimum between luminaries not less than 3 lux.(3) Overhead Signs. The level of lighting required for easy recognition and good legibility of overhead signsdepends on the ambient luminance behind the sign. There is no approved method for determining ambientluminance.The following explanations of low, medium, and high ambient luminance apply in selecting the recommendeddesign range of average maintained illuminance levels for signs.Low Ambient Luminance. Rural areas with no lighting or with very low levels of lighting. This wouldinclude background of mountains, deserts, fields, trees, and rural roads.Medium Ambient Luminance. Urban areas with small commercial developments, lighted roadways, andlighted intersections.High Ambient Luminance. Areas with high street lighting levels, and brightly lighted advertising signs.An expressway through or adjacent to a highly developed downtown area could experience high ambientluminance.When warranted, illuminate overhead signs to the following levels of average maintained illuminance:Ambient Background SignLuminanceIlluminance(lux)Low 100-200Medium 200-400High 400-800Maintain a minimum to maximum uniformity ratio of 1:6 over the entire sign surface.While the recommended values will provide good sign recognition and legibility for painted or enameled signsurfaces, the highly retroreflective sign materials require special care. In this case use luminaries especiallydesigned for these materials, and strictly follow manufacturer's installation recommendations. In addition, afield test of such equipment before accepting it for construction is desirable.(4) Rest Areas/Parking Lots. Rest area lighting installations shall not adversely affect the vision (by glareor spill light from the rest area) of traveling motorists along the main roadway. Motorists on the highwaypassing the adjacent rest area should be able to discern any vehicle leaving the rest area as well as the traffictraveling along the main roadway.9 - 94