Creative Director John Garvin shared with IGN the intricacies of the zombie horde's behavior on Days Gone.
The developer said that the crowds of phreakers moving around the map in numbers from 50 to 500 are not just cannon fodder, but a reasonable and cohesive organism: during the day they can hide in buildings and hibernate, and at night they go hunting together. Not all hordes like to migrate: there are those who lead a sedentary lifestyle and wait for the player in separate locations.
Eric Jensen, who is responsible for the design of the open world, said that mutants will react to the movement of the protagonist, following him even to the most inaccessible territories. Upon contact with the character, the horde will try to scatter across the entire width of the location in order to flank or hide before a joint attack. Frontal assaults are extremely rare for gamers to see.
For more details, check out this video produced by IGN.