When you can run around the world in less than a second, even the largest city would have to seem small and boring. So how does the Flash handle it? Easy, he lives in Central City, the city with no limits, 10-lane streets, and giant sidewalks.
The Absorbacon has an funny pictorial review of how the artists handled drawing a city big enough for The World’s Fastest Man.
Just imagine the run-off from rain over all that pavement. Of course, I doubt weather systems usually affected the entire city at once. :)
Update: More Central City, and it’s even bigger this time.