It's been a while since I've read the rules, but I don't think any terrain normally slows movement- at least not in a direct fashion. It imposes a command penalty, so you may end up stuck in a forest with poor order rolls.
If you play that terrain slows movement, I'd say that you ignore all terrain benefits and penalties for the purposes of determining routing. In other words, unless the dice roll higher than the base movement of the unit, it's fine. Similarly, if a unit is fleeing down a road (and you play that moving along a road adds to movement) that extra movement is ignored when determining if they route.
Personally, I wouldn't consider a road defended, but as long as you and your opponent agree then go for it.