I'm thinking predominantly urban area, probably at least 100k people in a non seasonal capacity. Eg. Pullman Washington is a small rural town that becomes huge during college and quiet in the summer as the students leave.
I tend to think of city limits as the boundaries of a city -- not the broad metropolitan area. Shorline is not Seattle, neither is Bellevue or Renton.
Edit: That said, we used to have streetcar suburbs as a prevailing model. Spokes that radiated out to the suburbs on relatively fast, uncomfortable streetcars. A similar model with trams or BRT could function.
Okay. The type of city I'm talking about would be like Aurora, Colorado.
I'm specifically not making any claims about cities like Seattle, Minneapolis, etc. Other than to say I think most of them already have all the transit that is economic for them to construct given each city's layout, makeup, climate etc.
Taking Aurora[1] there are almost 400,000 people and a median car ownership of 2 per household. This is the type of city where a lot of Americans live, and where no amount of BRT, Light Rail, etc. will convert them to a place where people can rely on transit for most of their trips, or even to decrease that car ownership figure to one per house.
[1] https://datausa.io/profile/geo/aurora-co/