An illustration of the space principle as it relates to cars
So today's Post Metro section has a great photo of cars traveling on Leesburg Pike through Tysons Corner, taken by Jahi Chikwendiu.
There are three lanes of traffic. Where the red line is that I have added to the photo is about how many cars in each lane are equal to the capacity of one bus. (I actually understated this number in favor of cars, because the likelihood of these cars having 1 or 2 passengers is pretty low.)
3 buses (40 feet each) or 90 cars.
A 60 foot bus would carry more passengers and "replace" more cars.
40 foot bus (about 60 passengers)
60 foot bus (about 90 passengers)
WMATA subway car (75 feet, up to 150 passengers)