Why is there no charge to travel on roads?

And another question.  Why haven't we asked this question? (or asked it enough).  Especially those of us in the green movement.

When we ride public transit we are charged money every time to off-set the cost.  Why are those who drive on the roads not required to do the same.  And even more so why aren't those with heavier vehicles (such as SUVs) required to may even more as they do more damage to the roads.

In the environmentalist movement I think we tend to focus on the same things over and over, but there are so many for things we can be doing and should be demanding.

If the roads are free, than public transit should be also.  And if public transit costs money than it should also cost money to use the roads.