California, the Loma Prieta quake of 1989.
https://en.wikipedia.org/wiki/Cypress_Street_Viaduct
Every time that Japan or Korea gets a major shaker, they have a very good chance of having one of their multi-level highways collapse.
Even when they’re not collapsing, elevated roadways of this type have some serious safety issues such as evacuation during fires or large traffic accidents. Accidents on one level often shut down both levels due to leaking fuel, etc. Also, higher elevated roads have a much greater chance of killing people in accidents when their vehicles go over the sides.
Basically it’s a bad idea.