On my work commute it varies between 15mph average (winter, cold, wet and dark) to 18mph (summer, dry roads and daylight) over a 22 round trip.
At the weekend I get about 19mph on a 35 - 50 mile trip out into Kent (traffic makes all the difference)
My aim is one day, on either route (not fussy), to regist 20mph+ average speed.
M