For me it depends on why I'm riding, as well as the terrain, although the terrain is pretty much consistently rolling hills, with a nasty one stuck in every 10 miles or so, just to keep me honest.
If I'm just out for a ride, with no particular destination or time restriction in mind, I'll go 12-15 and enjoy the scenery.
If I'm passing through a park with lots of pretty girls, I'll go 5-7 MPH
If I'm going to work, I'll go 15-18, except on the big hills, where I'll go 4-6
If I'm coming home from work, I'll go 18-20
If I'm out for a century, I'll go 12-16.