Heavy vehicles (bus and trucks) do much greater damage to the roads. They start and stop more often, they put significantly more weight on the road per axle or tire (especially when fully loaded).

A good example of this damage is the rippling that occurs at intersections on bus lines and in front of many bus stops.

There is plenty of scientific data to support this opinion: I won't bother linking to it because I don't think data is going to persuade a flat-earther like yourself: buses good, cars bad!

If you really want to save the roads, then ban the evil Tri-Met buses and buy everybody a Prius.