Weird. In the trailer description it was the uninfected who were moving quickly en masse trying to get away from the zombies, and climbing over each other, etc. But in the trailer, it looks like that may actually be the zombies who are moving like that. Hard to get a good look at them in the footage.
In the book, didn't Brooks mention that the "fresh" zombies were capable of moving faster than the ones who's been zombified and started decomposing? Thought I remember something like that, but it's been about a year since I last read the book.