PLZ MAKE THE ZOMBIES WALK WHEN I SET THEM TO WALK

There is a ‘rage mode’ mechanic, where a zombie has a chance to become enraged (read: move faster) for a few seconds after they’re hit. The chance is dependent on difficulty, with no chance of rage mode at the lowest difficulty. This could be what you’re seeing.

Originally posted by Brevan: Also, when you get to a higher game stage more Feral (glowing eyes) and Irradiated (green) zombies will appear, and I'm pretty sure they'll sprint regardless of default settings (to be fair I've never tested this).

There are four settings you can adjust for zombie speed: day, night, feral, and Blood Moon. Now that you mention it, it's a bit curious that there's no control per se for irradiated zombies. Conceptually, they're meant to be harder than ferals. They seem to move at the feral speed, based on experience, but I can't say for certain that's always true.