Estimate how many traffic signals in any city? Please take an example of any city and tell how to do market sizing?

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Recent activity on May 06, 2019
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gari asked on May 04, 2019

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replied on May 06, 2019
McKinsey Senior EM & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 350+ candidates secure MBB offers


Feel free to take inspiration from my explanation of a similar question below.


"Estimate the number of sewerage hatchways in London!"


I would start by radically simplifying the setting via assumptions:

  • Assume sewerage hatchays are only located on streets
  • Assume a rectangular structure of the London street network with horizontal and vertical streets intersecting in a 90 degrees angle
  • Assume average distance of parallel streets of 200 metres
  • Assume London covers a geographical space of 12x12km
  • Assume there is a sewerage hatchay every 250 metres on average (4 per km)

The general calulcation then looks something like:

[Total length of London street network in km] x [Number of sewerage hatchays per km] = Total number of sewerage hatchays in London

Hence, you need to to calculate the total length of the street network first:

[Number of streets x average length] = Total length of street network

From our assumptions we infer that the number of horizontal streets is 12x5=60 (since there are 5 parallel streets per km). The same is true for vertical streets: 12x5=60. So the total length of the street network is (60+60)*12km = 1440 km.

Now inserting into our above calculation yields:

1440 km x 4 = 5760

So, given the above assumptions, there are roughly 5750 sewerage hatchays in London.

It is important to also stress that this result is equally sensitive to all assumptions (since all elements are connected via multiplications). Hence, changing an assumption by a certain percentage will directly translate into a corresponding percentage-change in the result.

Cheers, Sidi

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Anonymous A on Apr 19, 2021

Hello Sidi, Thank you so much for your approach. I have just one question: Why the geographical space of London is 'only' 12x12 km? Isn't the territorial extension London's much higher?

Sidi on Apr 19, 2021

Maybe. I never lived in the UK, so I don't know. But this is COMPLETELY IRRELEVANT. MBB firms are NOT testing knowledge! They are testing your THINKING! So what is assessed is the LOGIC, not factual knowledge which fills this logic. This is btw. what many many candidates don't understand, because they see the interview like an exam, where they have to give CORRECT answers. That's not the point for an MBB interviewer. And understanding this will serve you very well during your interviews. :)

Anonymous on Apr 19, 2021

Thank you so much for the fast reply! Have a nice day!

replied on May 04, 2019
McKinsey / Accenture Alum / Got all BIG3 offers / Harvard Business School


How would you approach it? Happy to provide a feedback

Clue: Think of the area of a city, avg. area of a block and lights per block. Adjust the number to freeways, etc


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McKinsey Senior EM & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 350+ candidates secure MBB offers
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