Street lighting importance and design factors

Importance of street lighting

• It improves night visibility so that drivers see the road, obstacles, pedestrians and other vehicles in time to stop.
• It reduces crashes and the severity of crashes at intersections, curves, bridges, and pedestrian crossings.
• It improves personal security and reduces crime in urban areas.
• It increases the capacity and speed of the road after dark by raising the confidence of the driver.
• It helps traffic flow and the economic life of commercial areas (night shopping, tourism).
• It gives guidance to the driver on road alignment, junctions and the edge of the pavement, and it enhances the look of the city.

Factors affecting street light design

1. Type of road and traffic volume: arterial, collector or local street decides the illumination level (lux) and uniformity (about 30 lux on important roads, 15 lux on secondary, 8 lux on residential roads in Indian practice).
2. Mounting height (H): usually 8-10 m on main roads, 6-7.5 m on minor roads; higher poles give more uniform light and reduce glare but need more power.
3. Spacing (S): typically 3 to 5 times the mounting height; the spacing-to-height ratio controls uniformity.
4. Lamp type and luminous flux: LED, high-pressure sodium or mercury; wattage and lumen output.
5. Luminaire type, light distribution and tilt (cut-off, semi-cut-off, non-cut-off).
6. Arrangement of the lamps: single side, staggered, opposite, central (median) or suspended.
7. Lateral offset or overhang: the position of the post from the carriageway edge (about 0.6 m minimum clear from the kerb; 1.5 m or more on high-speed roads).
8. Uniformity ratio: minimum-to-average illuminance, about 0.3 to 0.4.
9. Glare control, maintenance factor (about 0.7-0.8), cost, local environment (trees, dust), and power availability.