Thanks for the feedback, one of the compromises was to just use wheel colliders instead of actual tracks for the digger, unfortunately these have a tendency to get stuck on things. One of the tricks is to use the bucket and boom to push yourself clear of obstacles, even just swinging it around often works wonders. With regards to the Zombie AI I must admit I spent a grand total of about 30 minutes on it so there's probably a few bugs, I do remember seeing what you described on the odd occasion however there were usually enough zombies to not notice.