- Lumen method: S = (Φ × UF × MF) ÷ (E × W).
- Φ = fitting lumens; UF = share of light reaching the road (often 0.35 to 0.5); MF = allowance for dirt and ageing (often 0.7 to 0.8).
- IS 1944 average lux targets: A1 30, A2 15, B1 8, B2 4.
- Common rule of thumb: spacing about 3 to 4 times the mounting height.
- Final spacing should be confirmed with a photometric layout (DIALux or similar) using the actual fitting's file.
Why spacing is where budgets are won or lost
On a 2 km township road, the difference between 25 m and 30 m spacing is about 13 poles per side. With foundations, cabling and fittings, that can be several lakh rupees. But stretch the spacing too far and you get a road that is bright under each pole and dark in between, which is worse for safety than a slightly dimmer, even road.
So spacing is not guessed. It comes from a quick calculation, then a proper layout check.
Step 1: Pick the target from IS 1944
| IS 1944 group | Typical road | Average lux | Uniformity (min/avg) |
|---|---|---|---|
| A1 | Important fast-traffic routes | 30 | 0.4 |
| A2 | Other main roads | 15 | 0.4 |
| B1 | Secondary roads with considerable traffic | 8 | 0.3 |
| B2 | Secondary roads with light traffic | 4 | 0.3 |
Step 2: Choose the arrangement
| Arrangement | When it fits (rule of thumb) | Notes |
|---|---|---|
| Single side | Road width up to about the mounting height | Cheapest; typical for village and colony roads |
| Staggered (zig-zag) | Width about 1 to 1.5 times mounting height | Better uniformity across the road |
| Opposite (facing pairs) | Width more than about 1.5 times mounting height | Wide urban roads |
| Central median, twin arm | Dual carriageways with a median | One pole lights both sides |
Step 3: Use the lumen method
The formula, in words: how much useful light one fitting puts on the road, divided by how much light each square metre of road needs.
Spacing (m) = (Φ × UF × MF) ÷ (E × W)
- Φ: lumens from the complete fitting (from the test report, not the chip rating).
- UF: utilisation factor, the share of light that lands on the road. Depends on optic, height and width; 0.35 to 0.5 is common for good road optics.
- MF: maintenance factor, for dirt on the lens and LED ageing. 0.7 to 0.8 is common.
- E: required average lux.
- W: width of road lit by that row of poles.
Worked examples
| Example A: village road | Example B: colony road | Example C: main road | |
|---|---|---|---|
| Road width | 7 m | 7 m | 12 m (opposite arrangement, 6 m per side) |
| Mounting height | 6 m | 6 m | 9 m |
| Fitting | 30 W, 3,600 lm | 30 W, 3,600 lm | 90 W, 10,800 lm |
| UF / MF | 0.4 / 0.8 | 0.4 / 0.8 | 0.45 / 0.8 |
| Target (IS 1944) | B2: 4 lux | B1: 8 lux | A2: 15 lux |
| Light-limit spacing | 41 m | 21 m | 43 m |
| Uniformity limit (3.5 × H) | 21 m | 21 m | 31.5 m |
| Use | About 20 to 24 m | About 20 m | About 30 m, poles opposite |
Look at Example A. The light is plenty for 41 m, but spacing that far on a 6 m pole would leave dark gaps. Two options: keep 20 to 24 m spacing and choose a smaller wattage (say 18 to 20 W), or use a taller pole. Both are cheaper than over-lighting the road.
Step 4: Check with a layout
The lumen method is for planning. Before ordering, ask the supplier to run a layout in DIALux or similar software using the real fitting's photometric file. The report should show average lux, minimum lux and uniformity on your road, with your pole height, arm length, tilt and spacing. Tenders increasingly ask for this.
Practical tips
- Put poles at junctions, bends, speed breakers and pedestrian crossings first, then fill in the straights.
- Keep poles clear of gates, drains and underground services; adjust spacing locally rather than blocking a driveway.
- On curves, reduce spacing and put poles on the outside of the bend.
- Avoid placing poles where tree canopies will grow into the light.
- Keep the same spacing along a stretch for a uniform look, especially on decorative projects.
References
- IS 1944 (Parts 1 & 2): Code of Practice for Lighting of Public Thoroughfares — Bureau of Indian Standards
- CIE 115 — Lighting of roads for motor and pedestrian traffic — International Commission on Illumination (CIE)
- IS 10322 (Part 5/Section 3) — Luminaires for Road and Street Lighting — Bureau of Indian Standards
Frequently Asked Questions
What is the formula for street light pole spacing?
Spacing = (fitting lumens × utilisation factor × maintenance factor) ÷ (required average lux × road width). Then check that spacing is not more than about 3 to 4 times the mounting height for good uniformity.
What is the spacing for 6 m street light poles?
Usually about 18 to 24 m on a straight road, depending on the fitting and road width. Confirm with a photometric layout.
When should poles be staggered instead of on one side?
As a rule of thumb, one side works when road width is up to about the mounting height; staggered suits widths about 1 to 1.5 times the height; opposite pairs suit wider roads.
क्या ज़्यादा वॉट लगाकर पोल दूर-दूर लगा सकते हैं?
पूरी तरह नहीं। ज़्यादा वॉट से औसत रोशनी बढ़ती है, पर पोल बहुत दूर हों तो बीच में अंधेरा रहता है। दूरी पोल की ऊँचाई की 3-4 गुना तक ही रखें।
DIALux लेआउट का मागावा?
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