The reality is that no major city in the world could be evacuated in a matter of hours. You just can't get that many people jammed through a couple of main highways that fast. The first car to have a flat tire, engine trouble, or heaven forbid a wreck would bring traffic to a stand still.

If a flank collapse in the Canary Islands caused a significant tsunami, many cities on the East coast of the US would be devastated... with a large number of people still there when it happened.