15 passengers on average seems way higher than most buses I've been on. Maybe during the very busiest times, but buses run all day. The many hours they spend with just four or five people aboard will really tank the average.

I don't wanna say there isn't busses that might producre more microplastic but an average of 15 passengers per bus isn't like some insane goal to achieve, even in the current world, especially once you factor in that there's also times that there's way more than 15 people on one for a given trip