If there ever was a commute which one could average over 20 MPH it's mine. I leave the house and ride on dedicated bike lanes/MUP for all but the last three blocks of an 8 mile commute. All major intersections have underpasses or over passes except one.

What's more is the bike lanes/MUP follow exactly the path of the Spokane River so there is very little elevation gain or loss. The steepest part is literally an MUP overpass over a busy street. Easy to sprint up because it's so short.

I really need to take a go pro video and post it here.

(FULL Disclosure - in these nearly ideal conditions I only manage to average 14 MPH but that's partly because I elect to ride a single speed bike where my top end speed is a little limited by how fast I can pedal)