Why should units have to make way for each other on roads anyway? Are these roads one man wide? Units can occupy the same tile without being in the same army, even if they're at war with each other. I've seen it with my own eyes. So why shouldn't they be able to walk through each other?

This troubles me as well.  Sometimes its just a frustration.  But other times its painfully costly... can't get reinforcements to a besieged city in time etc.  It's usually those damn merchant caravans that are getting in the way.  I can keep borders closed, but that doesn't stop the caravans.  Sometimes I force them off the road and harrass them from returning to it.  This is a pain in the ass to manage and unfortunately doesn't result in a new road being built.  And I've tried, but been unsuccessful in creating double-wide roads, and alternate paths... but the new carvans will initially go out of their way in order to hook up to an existing road.  And when I manually move a new caravan to its destination, the road frequently does not follow the route I proscribed.  Oy.  I wish, at the very least, that caravans could share tiles with opponent units.  Would be cool if all friendlies could.  I also wish the player had a bit more control of road building.