you are right to bring up speed as a factor. one interesting study found subway escalators are more efficient when people stand close compared to walking up! Sp

you are right to bring up speed as a factor. one interesting study found subway escalators are more efficient when people stand close compared to walking up! Space trumps speed in this case. In urban environments, cars have a low average speed (e.g. in London UK it's around 14mph) so I think space will dominate here too. Also remember that as speed increases, so does spacing for safety reasons. Cycle infrastructure should be built by taking space from cars, not busses.