Not much. I have 4 miles of gravel with no stops, then once I hit pavement it's 4 miles straight, a stoplight I usually make, then 3 turns only a block apart and 2 stop signs, then straight 2.8 miles until I get to work with no stops (one light but it's almost always green on the road I'm on. There's no way that the slowing down I do in town across the course of 3 minutes affects my overall speed much at all.

Out of a 40 minute commute I spend about 4 minutes not cruising on straight road. I spend a lot more time on hills, and for that I'd LIKE to have more spinning inertia; it wouldn't affect me grinding up the hills at 10 MPH that much, but it might store up some power to help me part way up the next hill.