The Three Goals of Road Lighting
IS 1944 and IRC SP-72 define road lighting objectives as: (1) Visibility - road users must be able to see obstacles, pedestrians, and road features clearly at a safe stopping distance. (2) Safety - prevent accidents caused by inadequate light or excessive glare. (3) Comfort - lights should not cause discomfort to drivers (blinding glare) or residents (light trespass into homes). A technically correct road lighting design balances all three goals simultaneously - not just maximising brightness.
Key Design Parameters
Maintained average horizontal illuminance (Eavg): the time-averaged lux level measured across the road surface at working plane height (0.85m). This is what IS 1944 specifies by road category. Uniformity ratio (Uo = Emin/Eavg): how even the light is. IS 1944 requires minimum 0.25–0.40 depending on road category. A uniformity ratio of 0.25 means the darkest point has 25% of the average lux - still reasonably safe. Below 0.20 creates uncomfortable bright/dark cycling for road users. Glare control (threshold increment TI%): a measure of disability glare to drivers. IS 1944 limits TI to 15% for major roads and 20% for minor roads. Achieved by proper luminaire shielding and mounting height.
The Design Process - Step by Step
Step 1: classify the road category (IS 1944 categories P1–P5, M1–M3, or C1–C4 for conflict areas). Step 2: determine target Eavg and uniformity from the table. Step 3: choose pole height based on road width (H = 0.6–0.8 × W). Step 4: choose arrangement (single side, staggered, opposite). Step 5: select luminaire - get the IES photometric file from the manufacturer. Step 6: run a lighting calculation in DIALux (free software) - input pole height, spacing, road width, and luminaire data. Step 7: read results - if Eavg and Uo meet spec, the design is complete. If not, adjust wattage, spacing, or pole height and iterate.
Quick Reference - IS 1944 Category Summary
Category M1 (motorways, expressways): Eavg 20 lux, Uo ≥ 0.40, TI ≤ 10%. Category M2 (main urban roads, NH/SH sections): Eavg 15 lux, Uo ≥ 0.35, TI ≤ 15%. Category M3 (secondary roads, MDR, ODR): Eavg 10 lux, Uo ≥ 0.30, TI ≤ 20%. Category P1 (pedestrian conflict zones, city centres): Eavg 15 lux. Category P2 (low conflict urban): Eavg 10 lux. Category P3 (rural through roads): Eavg 7.5 lux. Category P4 (village roads, residential): Eavg 5 lux. Category P5 (village lanes): Eavg 3 lux. Category C1 (signalised intersections): Eavg 50 lux. Category C2 (at-grade junctions): Eavg 30 lux.
