I wondered about this a lot when I got back on a bike. Then read here, where a lot say "Who cares anyway?" Which is entirely valid in my view.
The only reason I paid it any attention was I had a desire to try a club ride and a lot of clubs differentiate their different groups on speed, so despite the fact it's probably ultimately pretty unimportant, if you want to know whether you are fit enough to ride with a group it still seems to be the standard measure. To be fair, it's hard to think of a simple alternative.
Part of the trouble is there are so many variables. Wind, traffic and traffic lights are those that I found make the most difference. Road surface makes a surprising difference for me as well, maybe less to those who don't mind powering over manhole covers and generally uneven surfaces. So even on a given route I can vary by probably 5mph on any given day due to these factors before you even consider how energetic you were feeling.
Unless it's a hilly route, weight isn't as big a factor as you might imagine for us mortals. The basic premise is on the flat it's power and on a hill power to weight, thus on a hill you want the lightest combination of rider and bike and on the flat the most aerodynamic.
8 months ago my average was about 12mph on local roads, now it's more like 14, but even that isn't inordinately useful because it can vary wildly on a daily basis. On some smooth flat I can do (as
@Globalti suggests above) 20mph for quite a while, on some shoddily surfaced road into the wind and stopping at some lights along the way it might be 9mph.... And up hills? Depends on my best walking speed :P
17.5 over 7 miles without any other information, is as stated above, pretty quick I'd have thought.