Buses? Where? In the US they are as rare as unicorns. It depends on location. In third world countries metros, it is best to run electric buses for free and tax cars. It will reduce traffic and avoid hundreds of billions of road expansions if everyone is driving a car.

Anecdotally on my own US highway commute, the ratio of public-buses to large trucks is very small, in the range of 1:50 to 1:100.

The thing I'd like to highlight is that each category has a very different profile for which roads they use, and how heavily.