The time to think about getting out may be right now. If a large assault force is already within a city, making a run for it may be pointless. Air patrols or armored vehicles would just take you out for practice.
I have been putting a bit of thought into this eminent possibility. Maybe some longer term planning and action is in order?
If the SHTF in a pronounced major way, NO trucks will be bringing food to local stores. The city water will be turned off. Electric power facilities will shed residential and most of the other loads off of the grid. No gas, electricity, food, water or, medicines.
The real time to make a move might be now instead of when the SHTF! Be in a position to do and provide for yourself. Unless the upset period is a month or less.