The Zeds spawn in a different way than AI. If I'm not mistaken, they only spawn when in vehicles since DayZ 1.7.6 or 1.7.5.1 (can't remember).
If you want to try an updated SP : I suggest
DaiZy Factions or
DaiZy BP.
Factions is like vanilla DayZ, with more skins, a self bloodbag script, an improved building system and bandits as well as survivors. BP is based on Breaking Point DayZ (more skins, vehicles, weapons; harsher environment etc); it doesn't have survivors at the moment.
Of course, both missions have everything set regarding the spawns while driving.