There are some nice ideas here, but in zombie games we are aiming for the humans to "group together" or "help each other out" rather then punish them for it. Anyone watch resident evil? If you payed attention to those movies, you would realize a mechanic in them is that zombies have a sort of "human radar" as such. That is, the longer one stays in a certain place, the more zombies swarm over their establishment. This is because they sense the humans, and move to their location. therefore moving constantly is the best option, which does not discourage grouping together (just discourages staying still).

Nothing else needs to be changed, just use that mechanic for the problem.