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.
Buses also have more tires than cars – usually at least 6, but sometimes 10 or more. I still doubt they're emitting more microplastics than cars per trip but the math isn't so simple.
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.
Buses also have more tires than cars – usually at least 6, but sometimes 10 or more. I still doubt they're emitting more microplastics than cars per trip but the math isn't so simple.