interestingasfuck

Some places yes, keep in mind, some of these routes have been in constant use since the "track" era...

And especially the Roman roads have very well constructed foundations. So they were perfect for building right on top of.

often, yes.

Good question. Not in America, they usually just started from wagon tracks at best. I guess it depends on if they tore up the old roads first where they existed. Sometimes they used to integrate materials from the old road or nearby destroyed structures (after a disaster) as part of the new road, though not so much these days.