We used to travel a lot this time of year. We always allowed at least an extra day. When traveling in the coach, we always departed with provisions for three days.
Why Three Days?
Several times the weather turned out worse than was predicted. So, we just found a place to hunker down, stay warn and watch the chaos around us. We were held up for a big part of a day twice. But both of those could easily have become two days. So, the third day is just in case we did get held up for two, it was still not a worry.
Matt