Roadway Lighting Design Directive

PURPOSE

This Mohave County Public Works, Engineering Division design directive specifies design guidelines, as prescribed in AASHTO Geometric Design of Highways and Streets, governing road and/or walkway luminaire design and placement.

ILLUMINATION AND UNIFORMITY

  • Minimum illumination level (average luminous flux incident on the roadway pavement within the calculation grid):
    • Local roadways traversing residential areas = 0.4 foot-candles / 4.3 lux
    • Local roadways traversing industrial/commercial areas = 0.9 foot-candles / 9.7 lux
    • Sidewalks traversing residential areas = 0.5 foot-candles / 5.5 lux
    • Sidewalks traversing industrial/commercial areas = 0.9 foot-candles / 9.7 lux
    • Careful consideration needed if area influenced by pedestrian/vehicular volumes, night-to-day crash ratios, roadway geometry, merging lanes, curves, and intersections
  • Illumination levels at intersections shall be the sum of illumination levels on intersecting streets at the intersection.
  • Uniformity of lighting represents an indication of the quality of illumination and should be considered along with illumination levels. Uniformity of illumination can be represented by a uniformity ratio of the average-to-minimum lux values (average luminous flux incident on the roadway pavement within the calculation grid to minimum luminous flux incident on the roadway pavement at any point within the calculation grid) on the roadway or walkway surface. Recommended uniformity ratios:
    • Residential roadways = 6:1
    • Commercial roadways = 3:1
    • Residential walkways = 10:1
    • Commercial walkways = 4:1

DESIGN AND INSTALLATION

  • To minimize the effect of glare and to provide the most economical lighting installation, luminaries are mounted at heights of at least 30 feet. Lighting uniformity is improved with higher mounting heights.
  • Luminaire supports (poles) shall be placed outside the roadside clear zones. AASHTO specifies a minimum clearance of 1.5 feet between the curb face and obstructions such as lighting poles. For non-curb roadways, a clear zone commensurate with the road cross-section and design speed should be provided, as described in the AASHTO Roadside Design Guide.
  • Breakaway poles should not be used on streets in densely developed areas, particularly with sidewalks.

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