I would like to suggest some sort of animation from where the zombies spawns.
The spawn out of no-where is really weird (at least to me), and it would help by giving the humans a little headstart, so they know where they come from, and route around the zombies.
I was thinking something like, a dirt animation at the spawn place, and then make the zombie crawl up from a hole.
Ofc, this is just a suggestion. Thanks for reading.