The Spacing-to-Height (S/H) Ratio
The fundamental rule: spacing (S) should not exceed 3.5 × mounting height (H) for good uniformity, and should not exceed 4 × H for the minimum acceptable uniformity. At S/H = 3: excellent uniformity ratio (~0.40+). At S/H = 3.5: good uniformity (~0.30). At S/H = 4: minimum acceptable uniformity (~0.25). For a 6m pole: maximum spacing = 6 × 3.5 = 21m for good lighting, or 6 × 4 = 24m for minimum. For an 8m pole: 28m good, 32m minimum. These are starting points - actual spacing depends on lumen output and road width.
Spacing by Road Type and Wattage
Village lane (20W, 5m pole): 15–18m spacing. Colony road (30W, 5m pole): 18–22m spacing. Colony road (40W, 6m pole): 22–26m spacing. Municipal road (60W, 7m pole): 28–32m spacing. State highway (80W, 9m pole, single side): 30–35m spacing. State highway (100W, 9m pole, staggered both sides): 35–40m per side. Industrial road (60W, 7m pole): 25–30m spacing. These are practical ranges for Indian conditions - verify with photometric calculation for exact values.
Single-Side vs Staggered vs Opposite Arrangements
Single-side (all poles on one side): simplest, lowest cost, best for narrow roads up to 5m wide. Staggered (alternating sides): good for roads 6–10m wide - each pole illuminates the far half of the road at an angle, improving uniformity. Opposite (both sides aligned): used for wide dual carriageways (12m+) or when very high uniformity is required. Opposite arrangement uses twice the poles but gives uniformity ratios above 0.50. IS 1944 recommends staggered for roads 6–9m wide.
Quick Estimate: Number of Poles for a Road
Formula: number of poles = (road length ÷ spacing) + 1 (for the last pole). Add 10–15% for bends, junctions, and entry points. For a 1 km road with 25m spacing: (1000 ÷ 25) + 1 = 41 poles. With 10% buffer: ~45 poles. For a 500m road with 20m spacing: (500 ÷ 20) + 1 = 26 poles. Important: always illuminate both ends of the road, school gates, community halls, and water supply points independently of the regular spacing - add these as fixed points and fill in between.
