here's how I would approach it
- Define the geographic area to be covered [the whole world? the U.S.? NYC? Manhattan?...for this example I'll use Manhattan]
- Population living there [~ 1.5 M] + population travelling there for work or leisure [not sure about the figure, anything in the 50 - 100 M a year range]
- Divide the travellers figure per days of the year [100 M / 365 ≈ 275k]
- Assume a very low number of travellers comes to Manhattan by car [I'll say it's 0%] and most of the local population doesn't have / use the car [10% of people own a car and use it daily]
- The potential market for your service will be [275k] + [1.5 M * 90%] = [1.625 M]
- Assume [50%] of these people use public transportation, [20%] uses taxis and another [20%] either walks or bikes: this leaves you with [10% ~ 163k] potential daily customers
- Assume there's  competitors in this market with an equal market share: your company will process [10% of the transactions ~ 16k transactions a day]
All of the assumptions I used can obviously be challenged and can also be addressed by specific questions to the interviewer.
Hope this helped.