living in the DC area, I concur that the mpg would take a hit in the commuter traffic here. Luckily I can take metro to work, which is 35 minutes door to door for me daily.
My M3 gets a workout on the weekends and I average 14 mpgs in mixed city/hwy.
Recently I made a trip out to Fair Lakes and averaged between 22-23 mpgs. It's all about how you drive the car. Of course in my trip, the hwy was wide open so there was no stop and go traffic.