Interesting question! If you actually want to learn how to do these kind of estimations, try to come up with some structure to do the estimation, there is a lot of learning in that compared to just reading other peoples thoughts.
Some important steps I would consider are:
-How many people are currently using public transportation?
Linked to that are some important subquestions: What are the modes of transportation? Why are they taking these modes of transportation? And how does the demand vary over the day?
-How many self driving cars do we need to meet this current demand?
How many people fit into self-driving cars? Do we also have self-driving buses? Are people willing to rideshare? And maybe most importantly how can we make sure that the capacity is sufficient to account for peaks in demand (e.g. rush hour)
-Do we need to factor in that the availability of self-driving cars will change the behaviour of other people? E.g. If I used to ride my bike, I might now take a self-driving car to work
Hope this helps!