I guess the first question I have is where would they be trying to go and what would be there goal with traveling north or south? Are you thinking that they may try to immediately go to Castle Ravenloft? Because that would actually be an interesting choice and could work really well to set the tone for the adventure.
One of the problems with Curse of Strahd is that they portray Strahd as a cartoon villain doing evil for the sake of evil, and somehow every villager knows that Strahd is an evil vampire. But that's not how Strahd has traditionally been portrayed, particularly in the novel I, Strahd (required reading I think if you want to really get a picture of who Strahd is and his relationship to Barovia). In fact, few villagers see Strahd as anything but a distant ruler of Barovia. Few people interact with Strahd, and those that do aren't usually aware of it because he uses his alter ego, Vasili von Holtz. There are rumors that Strahd is not wholly natural, since he has ruled over the land for hundreds of years. Strahd rules with an iron fist and often personally doles out punishment for those that break his laws. But he is also a kind of protector of the land. He has been known to defend Barovia from outside threats, and he does have a kind of twisted sense of honor.
Anyways, I say all of this because if your players decide to just go right to Castle Ravenloft, Strahd may be willing to give them audience and even offer to become their patron. He may request the PCs to bring him Ireena Kolyana, who he believes to be the most recent reincarnation of his love, Tatyana. He may also charge the PCs with seeking out relics that could lead to his destruction, dealing with the rebellious were-ravens, or hunting down Rudolph van Richten. Over the course of these missions, the players may come to learn that their patron is actually a creature of evil intent and that they have been tricked into furthering his dark agenda.
A final thing to consider. The map of Barovia does not really extend beyond the border of the map. The entire region is surrounded by a barrier of fog. If the players do decide to go off road to, say escape Barovia by way of Mount Ghakis, they will encounter the mists that keep everyone trapped in Barovia. Also, as others have said, Barovia is a very dangerous place. And it gets exponentially more dangerous as you get out og the towns and off the roads.
This is a lot of info, and I'm not sure all of it is relevant to your question. But I hope it is helpful. Happy gaming!
