Well, I have a short commute also, with about 12 lights. I usually hit more than half of them green.
The real hit on MPG is when you are accelerating back to speed from a dead stop. That means every time you catch a red light, your MPG suffers A LOT. Even if you accelerate slowly, you are getting LOW numbers until you can assume a cruising speed. The fact that the car is not warmed up is also a culprit for hurting the morning commute MPG.
My best advice is to find two days who have nearly identical weather (use weather.com to find them) and do each commute one time with a cleared Trip meter and compare. If you hit 8 red lights on the city part of the commute, that's OK because that will be representative of what might sometimes happen.
I like to avoid the highway when I can because long stretches of 37-42
MPH seem to be best for my MPG in my 2004 manual tranny HCH, much better than 55-65
mph on the highway.
Good Luck !!!