larger regions with longer roads are hard to patrol, rebell armys could sit a lot of turns on the road, blocking your trades.
pretty useful in such situations are watchtowers which you have placed along the roads. they are really attractive to rebell armys and instead of sitting at your road the army occupies the tower, leaving the trade untouched (plus you "see" where a rebell army is because the watchtower is no longer yours).
this can buy you some time before your nearest governor/army is available to chase this brigands.