First let's think about some consumer choices that can affect our goals. Remember that people can choose to take transit for a single trip or as a habit. They can make an investment in it, and they can vote or lobby for a government subsidy to it. Similarly, people can choose to take taxis for a single trip or as a habit, or they can support government subsidies for taxis.
Thinking about it now, I've always assumed that a choice to take transit was a choice not to drive, but it's not necessarily the case. A person could choose to take transit instead of walking or riding a bicycle. Transit is not necessarily better for our goals in those cases; the rider may be paying a fare, but he or she may not be reducing the total pollution generated in the area. Similarly if someone takes a taxi instead of taking transit, walking or riding a bike, that's a net negative for our goals.
In the choice between taxis and private cars, there are several choices:
- To take a taxi instead of driving for a single trip.
- To take a taxi instead of driving for a habit or routine.
- To not buy a car and take taxis or transit instead.
- To subsidize a taxi system with curb access, law enforcement, roads, etc. beyond what is paid for the taxi permit or medallion.
Choosing to take a taxi instead of driving can reduce the need for parking if enough people do it, but the taxi driver may have to drive a significant distance more, to get to another fare, than the passenger would have if they had driven in a private car. It's probably good for the environment overall, but it's not clear how good.
Taxis have the greatest effect on the environment when they are a factor in a person's decision to sell a car, or not to buy one in the first place. To do that you need a reliable system where a person can expect a taxi to be available when they need one. Reducing car ownership is the critical role of taxis, and availability of taxi service is the critical factor in their ability to reduce car ownership.