Number of traffic lights in London- how would you calculate the number of intersections?

Market sizing
New answer on Jul 08, 2021
2 Answers
Anonymous A asked on Jul 07, 2021
  • Start from the London population = 8mn
  • Calculate the households estimating the n. of people per household (e.g. 2) = 4mn 
  • Calculate the number of residential buildings assuming the n. of houses in a building (e.g. 20) = 200k buildings 
  • Calculate the number of blocks estimating the n. of buildings per block (e.g. 10) = 20k blocks
  • Calculate the number of intersection ?
  • Calculate the number of traffic lights, assuming 4 main traffic lights per intersection + 8 pedestrian traffic lights (if needed to be included)


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Best answer
replied on Jul 08, 2021
Bain | passed >15 MBB interviews as a candidate

I think an approach not linked to population might work better. 

I would approximate London with 3 squares with perpendicular roads. THe numbers are very first guess and you might plug in different ones, but directionally should be OK:

  • An inner square with the city, marylebone, vauxhaull, etc...roughly 5x5km. In your approximation of perpedicular roads, you can assume a road every 100m. That gives you 50x50 intersections for that part.
  •  A middle square with the outer ring, residential areas, a bit less densly populated, area 15kmx15km, maby a road every 250m, gives you 60x60-20x20 intersections (take out the middle square)
  • An outer square with the suburbs, maybe 50x50km, an intersection every 500m, gives you 100x100-30x30 intersections

Note that now you'll still need to apply a discount factor, as not every intersection has traffic lights. Many of them have either a roundabout or no structure at all.

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Content Creator
replied on Jul 07, 2021
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Sorry, this is just wrong. Please don't default to population because that's what's easy/convenient.

The population of London does nothing to help you calcualte the number of traffic lights! Nor does the # of households.

Rather, do this a bit more "bottom-up". Think about an area or two of London that you know. Extrapolate out to how many traffic lights that area has. Then, multiply out based on how representative those areas are (in total size) compared to the rest of London.

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Henning gave the best answer


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