From a lecture I gave at a university in 2012
If you were a pilot in the Military and your plane or helicopter had just been shot down and somehow you were lucky enough to have survived that initial trauma, that is just the beginning of the crisis you now face. You will need to get the hell out of the AO or area of operations, better known as the immediate area. The reason quite simply is because the enemy saw you being shot down and will be looking for you. You need to put distance between you and the enemy, otherwise you will end up as a prisoner of war or more likely they will kill you.
Now in the civilian world and in whatever crisis does occur you will need to assess the situation to determine if you should shelter in place or if evacuation will be necessary. You will make the decisions based upon the circumstances and the threat. If an asteroid has hit the earth and has not landed on your house, then shelter in place could be the best option. However if you are in low land area and a threat of flooding exists, then you may have no choice but to evacuate immediately. If you have an invading foreign army, again evacuation to a secure area may be the only option for you and your family. Hopefully you have already prepared for either case by accumulating reserve food and supplies for either scenario you and your family may be faced with, rather that be shelter in home or as a last resort evacuate to wherever may be a safer place. You must have a pretty good idea already as to where might be a safer place in the event of such an evacuation and also the travel route necessary. Hopefully you have already accumulated a number of road maps and in addition to main route you would want to take, you should also have alternate route already in mind.
Many people have developed a habit of keeping their vehicles filled up at all times, simply in case they do have to evacuate. Many people like me, who drive pickup trucks already keep secured in the bed of the truck, a number of 5 gallon cans at the ready. Others may secure their reserve supply in their garages. Some people already keep loaded in their vehicles what is called a "bug-out bag" which already contains all of the essential supplies they may need, just in case such an evacuation becomes necessary.
The name of the game is “Be Prepared for the worst and hope for the best. If you wait until the last minute and then decide their was something you overlooked and forgot to include in your bug-out bag” or stashed away for use in this emergency, it will most likely be too late to acquire it.