Tripping will add a interesting new element and make panic more of a thing than it currently is. As long as the tripping mechanics are balanced and don't occur too randomly. I'm guessing that it will have to be based on random probabilities coded into the game that change with conditions like weather, terrain type your running on, moodles etc. With this in mind I'd say that whilst the weather's fine and your suvivor is running along happily the chance of tripping should be irrelevantly small. As conditions worsen this goes up until you find yourself slipping in the rain or falling down in a panicked, overburdened, exhausted state. Add the difficulties of the ever encroaching wilderness (tripping over bushes and in long grass) and use the nimbleness skill to improve the suvivors chances of not tripping; should be an interesting addition. Looking forward to it!