> 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.
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.