If you have enough density to justify a bus lane, you have enough density to justify a subway.
That assumes a linear city, where everyone lives within a short walking distance of the same street.
In actual cities, bus lines from different neighborhoods converge on main streets. While individual lines may have 10–15 minute intervals, bus traffic on the main streets may be high enough to justify dedicated bus lanes.
Then, as the city grows, it can make sense to replace the bus lanes with light rail and direct bus lines with collector lines connecting to the rail line. Which should be cheap, as a dedicated lane is usually the most expensive part in building light rail.
But you generally want to avoid building subways until you have no other options left. Subway lines tend to be an order of magnitude more expensive than light rail lines. Travel times are also often higher, as the distances between stops are longer and there is more walking involved.
Not at all. Building a subway in most US cities right now is very expensive. Raising the tax revenue alone is probably a non-starter.
Moreover you're going to have to close the road down anyway to do any form of cut-and-cover or even deep bore construction, which means every business on the corridor and every person who lives on it is going to get angry for as long as the subway is being built.
There's no painless way to do infill public transport. The problem is that nobody in the US is willing to compromise.
This is true but seems like a problem worth solving. It's also true of more than subways; we have the same problem with bridges, housing and many other things. Better to get on with fixing it than use it as an excuse for doing something worse.
> Moreover you're going to have to close the road down anyway
That's a one-time cost, and you're not required to close a 500 mile stretch of road for years on end. Dig one block, install the tunnel, cover it, dig the next block.
IMO it comes back to the fact that Americans are just not willing to accept change of any kind right now. The economy feels too shaky, the electorate too divided (even within states and municipalities), and there's too little faith in government to architect the kind of change you'd need to build subways, underground metros, or even BRT. We need a larger feeling of unity even at a state level to make the changes necessary, which is why municipalities continue to do bare minimum maintenance of roadways and pretty much nothing else. The last big set of constriction in dense urban areas was funded by the Obama stimulus from the GFC which was passed 17 years ago.
Only if you're also intentionally making point-to-point worse.
Note that I'm not comparing to "get in your own car and drive", which has the disadvantage of having to park. I'm comparing the ideal taxi-shaped thing to the ideal bus-and-tram-and-train-shaped thing.