This brings us back to internal region features. Thinks like
towns (that is the market etc) should be able to have limited
access. This means you can control who gets to buy and sell,
but is kinda realistic (does "you earn 20 gold pieces building
town walls" ring any bells?).
Routes should only be able to be blocked at the termination points.
Thus you may travel for 20 days only to be turned back by the town
guards. You shouldnt know in advance about them. This means your
going to need to differentate being at the border of a province (which
should still let you use the exits normally) from the interier features
(which may have exit / entrance restrictions).
Places like passes where it woudl be feasable to have restrictions
on the entire province, and going to cause headaches though.