The towns around Chernoble
have been abandoned for quite some time, which should satisfy your curiosity about how real cities decay if suddenly left alone.
Chernoble is also a great robotic testing ground because people still can't really go there for long periods, so no cheating.
The question of how cities would decay if humans suddenly died off en masse is moot. The reason is that before long, the world's nuclear reactors (especially the older designs) would start running out of coolant and going Chernoble themselves. Will Smith will not be fighting vampires 3 years later because radiation would have killed him already.
And, of course, we could use Chernoble for movie sets and kill off the god awful generation of actors that is plaguing American cinema.