Michigan is geographically two peninsulas. Most people live in the southern, or lower peninsula. The northern, or upper peninsula is much less populated for a variety of reasons.
|The Mackinac Bridge during the Labor Day walk.|
The two peninsulas are connected by the Mackinac (pronounced mac-i-naw) Bridge. Bridges are natural choke points. Fleeing toward the tip of a peninsula is akin to trying to avoid a sword fight by hiding on the end of a pirate ship's "plank". That is why south is the only rational direction for me, living in Michigan's southern peninsula, to "bug-out". If I lived in southern or central Florida then fleeing north would make the most sense for exactly the same reasons.
Expanding on the need for a destination: Nobody explains it more clearly than Duncan Long in his essay Backpack Fever. A conservative estimate for the pre-Columbian the aboriginal population of North America is seven million people (Russell Thornton). The forests and plains teemed with game, the streams with fish. The natives practiced agriculture and adeptly harvested and stored transient windfalls like migratory fish and nut drops.
A family might be able to "camp out" for two weeks based on what is in the trunk of their car. But they are not going to "subsist" based on small inconveniences like private property, degraded eco-systems, gross over-crowding and, frankly, lack of skills.
One hazard I did not discuss was chemical spills. Suppose a train load of methyl-ethyl bad-stuff derails, then what?
Nine-times-out-of-ten the best evacuation plan for a chemical spill is an in-home evacuation. In our mind's eye our evacuation has us whistling down the Interstate at seventy miles per hour. In actual practice you will be stuck in traffic or still looking for one of your kids when you get rolled over.
And don't expect to see that made-for-TV fog bank coming at you! You won't see it. You probably won't smell it. And some of these chemicals (Guthion comes to mind) can kill you so fast you will be looking out your windshield before you blink and looking at the Pearly Gates after you blink.
In house evacution, i.e., sheltering in place
Close your doors and windows. Duh!
Any device that blows air out of your house is also, invisibly, sucking in "make up" air. For the sake of all that is Holy, don't run the clothes drier. Turn off the thermostat so the furnace does not kick on. Don't turn on your bathroom exhaust fans.
Camp on the highest floor available to you and wait for Mother Nature to execute the dilution solution. This is a good time to listen to the radio and/or use your smart phone to monitor local news stations or city websites. Police scanners rock at times like this. Most methyl-ethyl bad-stuff has high molecular weight. Guthion, for instance, has a molecular weight of 317 compared to air's molar mass of 29. The bad stuff wants to "puddle up" in low places and depressions. Elevation is good.
If you have kids, you will be tempted to go out looking for them. Don't. Use your phone to find them. Then an executive decision as to whether to tell them to flee (which will be their natural inclination) or to shelter in place.
How about that one-time-in-ten?
How about that one-time-in-ten when fleeing is the right decision?
Most of those times involve chemicals of relatively low toxicity and high flammability. It may be very obvious when that river of flaming kerosene turns into your subdivision and starts heading toward your house.
Other times it will be a call by the emergency responders. The ruptured tank may leaking flammable liquids/gasses and they want to evacuate due to risk of explosion. The emergency responders may go door-to-door but you will know about it more quickly if you are monitoring the event. Hence the comments about the scanners and smart phones.