If a severe earthquake or flood were to occur in Tokyo, how easy would it be to evacuate the city? Answer: Not very.
A government panel is currently conducting a study on what would be involved in evacuating only 5 Tokyo wards in the event of flooding along the Arakawa river.(1) It estimates moving 1.5 million people out of harm's way would take half a day or longer.
But how about in the event of a Fukushima-like earthquake with a magnitude of 7+ on the Japanese scale underneath Tokyo? No one seems to have a clear answer. But if it would take half a day to move 1.5 million people, how long would it take to move 13 million (Tokyo proper) or 37 million (Kanto plain)?
To get an idea, all one needs to do is revisit the 2011 Fukushima quake to see the limits of what can be done. I was here during that event and witnessed for myself the following: Convenience store shelves were the first to go empty as roads and train service was stopped. In the ensuing days, store shelves around the city faced shortages of rice, milk, bread and eggs. Petrol stations experienced long lines with some running out of fuel due to lack of replenishment.
This is what happened with a large quake 100 kilometers from Tokyo. What would happen if roads/tracks were buckled or gas/water/sewer lines broken? At the end of the day, the system we use and depend on to live is precariously fragile.
In fairness to government authorities, there is a limit to what can be done. This was confirmed during the Fukushima earthquake where evacuation had to be weighed against the onset of sheer panic.
In any case, everyone who lives in Tokyo should have a personally-devised plan to get out should the need arise. A bicycle or motor bike could prove invaluable. Keeping the bathtub filled with water is another option some people I know do. I even have a friend who keeps dozens of dry food packs and bottled water stored at home. Good idea.
The BIG one will come. (2) It is a matter of time. Having Plan B ready to go makes good sense.
(1) https://www3.nhk.or.jp/nhkworld/en/news/20170213_23/
(2) https://ig.ft.com/sites/japan-tsunami/