Here is a question which might be silly. When and for what reason would anyone leave their home? Except of course if it was destroyed.
Surely your house is your most secure place.
When reading the novel I mentioned, it seems huge numbers of people are fleeing the cities very soon after terrorists knock out communication, air and railway systems. So the major roads are full of traffic. Why would people risk the greater dangers of the unknown by fleeing all they know?
Because most of the world lives in large cities....around 80% of the population. They are targets for invading forces. Also food production doesn't happen inside cities...processing and warehousing of foods happens in the suburbs. And cities need regular supplies of food and water. When the power goes then all fresh water, food, and sewer is gone.
So what ever shrinking supplies of fresh water and food that exists are going to become targets of those without any. Regardless of right or wrong....people will justify themselves into taking what you have by any means necessary. Scared, hungry/thirsty and desperate people who once were your neighbors are now potentially perpetrators of the worst things possible to you and your loved ones living with you.
Things are actually rather Rosie when you are well fed and not completely dehydrated....you ponder and desire toys and new things to hold your interests like hobbies. But when water and food are gone...your justifications for stealing and harming others so you can take their water and food seem so plausible that you commit them.
I know that this sounds horrible to say...because many people you know desire to be good and kind and generous.
People also panic....for doing essentially nothing more dangerous than walking and breathing, people who are panicking will attack you because even though you have done nothing to give anyone pause they will imagine danger and attack you.
So....to avoid having to fight on a daily or even hourly basis and get a good night's sleep....you get away from people.
Personally I happen to think that you are right.....wait until the people rushing out of the cities slows or stops and then leave. This of course requires provisioning. Water, food, and disposal of human waste and garbage developed that attracts no one. (Odors are an attractant)