You kids these days, everything with you has gotta be fast. Your internet connections, your high-speed trains... even your zombies. In MY day we had dial-up, and regular-speed trains, and SLOW zombies. And we LIKED it. You should be an old coot like me, and add slow zombies to State of Decay (along with other changes) using the George Romero Mod .
There are a number of things the George Romero Mod does, or can do, for State of Decay, and you have the option of choosing exactly which changes you'd like to make and ignoring those you're not interested in. First and foremost, you can use it to adjust the speed of the zombies. In the game, the default zombie speed is somewhere between the slow zombies of the original Dawn of the Dead (1978) and the Usain Bolt zombies featured in the remake of Dawn of the Dead (2004). The mod slows them down a bit, so they're more of the shambling, lumbering, more-staggering-than-sprinting variety, in keeping with the tradition of Romero's original vision of animated corpses.
Does this make it easier to avoid or escape from zombies? Sure, and I guess that may seem like a bit of a cheat, but the mod compensates for this by making zombies more numerous and a bit harder to kill. So, even though zombies are noticeably slower, there are more of them to deal with, and they're distinctly more durable. It's a fairly even trade-off, so it doesn't really make the game feel like it's gotten any easier, and even just those small changes make State of Decay feel more like a classic, Romero-esque experience.
In addition, the zombies' hearing and eyesight is also considerably better. Noises like hasty looting, and especially gunshots, will bring them lurching to your location more often and from further away. The beam of your flashlight, too, will alert zombies to a higher degree, and one optional feature of the mod, called Deadlight, can make even the noise of clicking your flashlight on and off attract the flesh-eaters.
Speaking of making things challenging, you can choose to enable Fatal Attacks mode, which essentially means that a single zombie bite will put your heath meter to critical, and two swipes will be enough to KO your character for good. I definitely recommend this: it makes every encounter, even with a solitary slow zombie, a true life or death encounter. It also makes it extremely easy to do away with a character you don't like!
And, since you're dying horribly anyway, you might as well be able to watch. Enabling the mod's Fulci Vision feature (named for gore-film director Lucio Fulci) means that when zombies rip you to shreds, the screen will not be obscured by the typical red haze. You'll get a lovely unobstructed view of your body being torn in half. Yay!
To help you survive the slower yet more crowded and deadly zombie apocalypse, you can enable the More Outposts feature of the mod, which gives you the ability to turn a bunch more buildings into safehouses (though these new safe spots won't show on your map).
There is one option of the mod that feels like a huge cheat: you can choose to turn off vehicle damage, meaning you can run over crowds of zombies all the dead-long day and not so much as smudge your bumpers. This is entirely optional, however, like all of the mod's options, and you can leave the vehicle damage as is or even choose to make the vehicles weaker for an extra challenge. There are so many drivable cars in the game already anyway, there's really no reason to make any car indestructible.
While all of these features can be used independently, you can also enable Dread Mode, which combines several of the features to emulate the George Romero experience: slower yet harder to kill zombies, gunshots and flashlights attracting more attention, and zombies doing massive damage to you. This mod was created by Adeno, who also created the George Romero mods for the Dead Island games, and it's still in a bit of a beta mode, though everything I tried seemed to work just fine.
Installation : Grab the latest version of the mod (v11 is what I used) here . Don't unzip anything yet, just put the zip file right in your State of Decay "Game" folder (Steam > steamapps > common > State of Decay > Game). Using WinRAR or WinZip, extract the downloaded file right there (right-clicking and choosing "Extract here" is an easy way to do it).
You'll see a bunch of new zip files in the folder: each contains a different element of the mod. Any file you want to use, just extract it right there. Full descriptions and instructions can also be found in a readme contained in the original zip folder.