Even though I now drive in bumper-to-bumper stop-n-go traffic every day for over half of my 17 mile commute to work, I still manage to get almost 18mpg out of my V8 4Runner at this point. I tried something one day. I held back in traffic (which thoroughly pissed off the guy behind me) a little bit. I'd let the cars in front of me get as much as maybe an 1/8th of a mile ahead in certain areas where traffic flow would get up to 30-40mph on the highway. In areas where it was down to 5-15mph, I'd only leave about 6-10 carlengths. The highway I'm on is only two lanes, so if you can get next to an 18-wheeler that's doing the same thing, it prevents people from zooming around you and screwing up the experiment. At any rate, the section of highway that's jammed up is about 9 miles long and I ended up only needing to touch my brakes 2-3 times during that entire distance. I would watch people in front of me hitting their brakes every 5-10 seconds, and I barely even touched mine. This helps your mileage and increases brake pad life -- no wonder some people only get 30k miles out of a set of pads... if they drive like some of these people, it's completely understandable.
The bottom line is, any of the traffic immediately behind me had a much nicer and less stressful commute if they stopped and realized what I was doing. I was smoothing out the flow of traffic. Any time traffic would come to a dead stop in front of me, I was already coasting and slowing down without the use of brakes so that by the time we started reaching the cars up ahead, they would start rolling again. Most of the time I would time it to where I'd get within a normal 'rush hour following distance' right before they'd start out-accelerating me again. I'd then let them go on ahead and get that 500-1000 foot gap and just roll along. Next time they stopped, I'd be right there again, all the while my speed never varied all that much, really.
It was an interesting experiment, pissed off a few ignorant people, and I have actually adjusted my driving accordingly. This is pretty much the only way I drive in rush hour traffic. I was surprised that even in a vehicle with an automatic you can do a good job of engine braking and maintaining speeds appropriately. For somebody looking for something to do (other than talk on a cell phone) in rush hour, it's something that will make you focus on driving AND give you better economy. The real challenge is to do it without downshifting manually for engine braking. Keep it in overdrive the entire time.
Ricky