alt Hacker News> 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.
Isn't that the assumption you're making? That there is a single primary street that everything converges and then diverges from which is common to every bus route? Meanwhile in practice any given person standing on the You Are Here dot could want to go in any of the eight directions from where they currently are.
A route that goes east-west isn't going to have much in the way of shared route with one that goes northeast-southwest except for the one point where they intersect, and isn't it better to have multiple routes intersecting in multiple places in terms of minimizing trip latency and maximizing coverage?
> Which should be cheap, as a dedicated lane is usually the most expensive part in building light rail.
But that's the thing that makes the bus lane so expensive!
By the time you have an area with enough congestion to be considering a bus lane, the problem is generally that you can't add a lane because the land adjacent to the existing road is already developed and not available, otherwise you would just add an ordinary lane that buses could use too. But converting one of the existing lanes in an area which is already congested makes the traffic exponentially worse than putting the new thing underground.
Essentially, if you can add a lane then you add an ordinary lane and if you can't add a lane but need one then it's time to dig.