These signs are generally found before a lower-than-normal bridge or overhang, or at the entrance to a parking garage; and as a tourist, unless you're actually driving a bus, these height restrictions shouldn't be of any concern to you.
Although, perhaps you should take note of how high those red double-decker topless Hop-on Hop-off tourist buses are - just in case you see one of these signs while out sightseeing on the top deck. ;-)