The only problem being that it isn't just one fatty that would be slowing other people down. I don't know fire codes, but let's pretend that in a fire, you have a stairwell allocated for every 1000 people in the building. Assume further, that 10% of the people in the building are super-morbidly obese and will slow people down. That's 100 lard buckets per stairwell. Even if you did put a bootprint on the back of all 100, you will have cluttered up the stairwell with 500-pound roadblocks to the point of making it nearly useless anyway.