Almost none. I live in the middle of nowhere, I hit about three stops before I hit highway and it's all highway after that.Stop and go is the differentiating factor. How much stop and go do you do ?
Hydrids do much better with more stop and go.
You get better mileage on the highway than in stop and go even with a hybrid. While the largest difference in increase from hybrid to a regular vehicle is in the stop and go, the hybrid gets better mileage on the highway than in city driving.