I find that hilly terrain does take my speed down 1-2mph (you make some time back coming down hill, but you dont make it all back). Of course "hilly" is very non-specific, so take my comment it for what its worth.....nothing.

Given my experience among the general biking public in my local area, 18mph+ pretty much will have you passing 97% of everybody on the bike paths and streets. The other 3% will be more experienced riders, and probably also people who are into cycling enough that they'd be posting on the internet more often than the slower 97%, making it look like everybody is faster than you. The reality is probably very different.

Anyway, if you really want to quantify your performance, get a power meter. It will tell you exactly how strong you are, and then you can look it up on the infamous chart in the power meter book to see how big your e-wang is.