The big difference is wear and tear. Some other article posted here a few days ago talked about this difference on road attrition. It takes one bicycle to go over a road 160 thousand times to cause the same damage to a road than a single car passing once.

So, building a smooth road for bicycles, which were lobbied for and constructed first for bikes waaaay before the first car was ever invented, has a lower cost and much better ROI than the same road being used by cars. Even in rough weather conditions. Also, I'm much more efficient on a mountain trail in a bike than any 4x4 truck.