They are, but for much lower passenger volumes than trains or subways. A bus rapid-transit line with all the bells-and-whistles like grade-separation (don't sh

They are, but for much lower passenger volumes than trains or subways. A bus rapid-transit line with all the bells-and-whistles like grade-separation (don't share any roads with cars), elevated platforms at every station for faster boarding, etc. can't handle more than about 3k passengers per hour[1]. A standard bus-line without that extra infrastructure, much less. A subway line can handle 30k passengers per hour. You could build ten times as many bus lines on parallel roads, but it's probably going to cost more than the subway line and you are going to run out of space when you consider locations that have a subway station every quarter to half mile. Fitting ten bus lines in that space would be one every 150 feet or so. [1] https://www.liveabout.com/passenger-capacity-of-transit-2798765 https://www.liveabout.com/passenger-capacity-of-transit-2798...