I quite like the system that games like A.D.O.M and Fallout 2 had, where one of your characters skills is the 'Survival' skill and the higher this is the more likely you are to be able to avoid random ambushes.
So say (for example) a pack of wolves has an 'ambush rating' of 20 then characters with a 'Survival' below 20 will always get ambushed by wolves and characters with a 'Survival' skill above 20 are presented with the option of either engaging the wolf pack or evading it.
So say (for example) a pack of wolves has an 'ambush rating' of 20 then characters with a 'Survival' below 20 will always get ambushed by wolves and characters with a 'Survival' skill above 20 are presented with the option of either engaging the wolf pack or evading it.