instead of posting small armies along a supply route, why not have small outposts, not quite villages or cities, for resupply and check-in along supply routes, which could send out raids on any bandits trying to attack a merchant/etc.
crap, sorry vigilus, didn't see your suggestion before I posted. My bad XD
still, i did add a little bit with sending out patrols (said "raids")
A cyber-optic snake (half-dracula)