Streetlights in a metro city
Road-length × spacing — geometry does the estimating.
Estimate the number of streetlights in a 600-km² metro of 10 million people. The road network is the skeleton: estimate road-length per km², then divide by pole spacing. Geometry beats demographics here.
Road length
Dense urban grids carry ~10–15 km of road per km² → 600 × 12 ≈ 7,000 road-km.
Split the mix
Arterials/highways ~15% (1,050 km), lit both sides at ~30 m spacing; local streets ~85% (5,950 km), lit one side at ~40 m.
Count
Arterial: 1,050 km ÷ 0.03 × 2 ≈ 70K. Local: 5,950 ÷ 0.04 ≈ 149K. Parks, junctions, flyovers ≈ +10K.
Total
≈ 2.3 lakh streetlights.
How to defend it
The road-density figure (10–15 km/km²) is the one unfamiliar number — anchor it physically: a 1-km² grid of blocks every ~150 m contains ~13 km of internal road. Deriving your anchor from geometry, on the spot, is exactly what this genre of guesstimate tests.