Alright, I've avoided these types of threads for some time but now have some actual Endomodo data to base my response on so here we go: 15mph average, 18mph high, 12 mph low.

Commuting is all about context. I would like to cite the following factors in my commute which is 4 miles via street and 7 miles via MUP, many luck based, others not:

1) Hitting the first light. Pure luck. Can add +30sec, more if you hit the sweet spot. On a 45 minute route = 1%, still something.
2) Stoptional signs. I'll admit I ride through 4ways at high speeds early in the morning when nobody is out.
3) WIND! Chicagoeans know what I am talking about
4) MUP traffic, plenty of folk hogging the trail.
5) Rest/dehydration/hangover... nuff said
6) Last lights leading to the loop.

So, my average speed is what I said above, 18-12, usually around 20mph on the MUP and 15mph in the streets for rested and decent conditions. All commuting time logged, 14.1mph average including critical masses, meh.