Honestly it's one of those things that when it happens you'll know. The situation that requires flight from your home is not just a singular event but rather a series of events that forces you to abandon your house for potentially safer harbors. A volcano erupting in California for instance has little impact on us in Virginia however, the ash cloud could cause the environment here to deteriorate to the point we may have to flee north or south. Bugging out is the last option and one I do not make any plans for since I can not imagine the exact circumstances that will occur when it becomes necessary. Instead I keep supplies that are easily movable and plan on the fly, look for oddball locations that only a few people would think about, chose my new location based on strict criteria and know that my training and knowledge will be more essential than any gear I own.