The environment is stressful.
The enemies in the open world randomly spawn. Iirc there were also animals, crows, bears and wolves. At one point, I was just merely going from point a to point b, and I was ambushed on the road by human enemies and it was an instant death. Then when I activated a fast travel (yes you have to do something just to activate fast travel points), two screamers came out and I was cornered. When they scream, they attract more zombies. And one time, I almost ran into a horde of zombies, it was crazy. And your bike runs out of gas, so thats another thing to always consider.