Depends on

• Total population density

• Travel habits of the people. Miles per day

• Mileage per vehicle – take terrain into account. 10 miles a gallon

• % gas vehicles = 80%

• % people driving cars = 60%

population density = total population / total area = 1M / 100sqmiles = 10k

Gas used per sq mile = # gas vehicles per sq mile * miles per day per vehicle / avg mileage.

= (10k * 0.6 * 0.8) * 20 / 10 = 10k gallons

#GS per sq mile = gas used / capacity per station = 10k / 10K = 1

#GS = GS per sq mile * total area = 1*100 = 100

Gas Stations = Refueling trips / Refueling capacity per station

High level approach (SWAG)

• Area of SF city = 7 miles * 7 miles —> ~50 square miles.
  • Area served by a gas station 2 sq miles (based on observation)
  • Total Gas stations = 50/2 = ~25

Bottom up approach 

• Residents in SF = 800K
• Households  = (800/3)  —> 300K
  • Cars = 450K (1.5 per HH)
• Gas stations
  • Avg # fueling pumps per station = ~6
  • Avg refueling time/car = 6 mins —> 10 cars an hour per pump
  • Per hour capacity = 60 cars per hour per station
  • Hours open / day = 16 hrs
  • Daily max capacity = 16*60 = ~960 —> 1000 refuels per day per station
  • Monthly capacity = 30000 refuels
• Cars refueling
  • Avg refueling needs = 2x per month
  • Total refueling = 450*2x = 900K refuels per month
  • Refueling out of city = 10%
  • Refueling in city = ~800K

Gas Stations = Refueling trips / Refueling capacity per station —> 800,000 / 30000 = ~26 gas stations

Things to watch out for

1. Assumption of number of refuels is highly sensitive to where people work and refuel.
2. Commuters coming into the city may be using SF based gas stations but difficult to model that

Clarifications: None

Equation:  SF Population* Gas required Per Month / (Average Capacity of 1 gas station * 30 days consumption)

Catalog I should know:

 SF population = 1 M

Average Gas required:  100 gallon per month (average miles per month 1000 miles and an average mileage , let’s say 10 miles per gallon)

Demand:  100M Gallon per month

Supply:

Average Capacity of 1 gas station: 12000 -25000 gallons = 18000 gallons (  whenever i go to a gas station there are 4-5 cars. So assuming in an hour 20 cars are served ( peak will be more and night will be less, so around 500 cars served in day, and average car/Big vehicle tank capacity will be 25  -50 gallons. As a gas station generally gets filled once in a day assuming capacity will be  : 12000-25000 gallons .

 Value: 100 M/20000 *30 days =100 * 10/6 = 170

Sanity Check

No of Gas station: :  SFO area/No of gas station per square Km

SF Area: 120 Square Km,

Let’s say  approx every 1 square km has 1 gas station. So  there should be 120 gas station

This is in the same range , so it should have around 120-170 gas stations.

Ans: 150 Gas station ( Keeping average)