I have a 25-mile commute. about 30 traffic lights in total, many, many junctions.

Normal elapsed average (summer) is 17-18mph. One day I had a howling tailwind and without breaking any traffic laws managed to average 20mph (elapsed). I was really caning myself tho' and like I said, howling tailwind.

Interestingly, I've done the route using a GPS as my 'cycle computer' a few times. There isn't much difference between my rolling average and my elapsed-time average. Probable because of my habit of easing off and coasting up to traffic lights that are red or about to change.