Please correct me if I'm wrong here. I'd assume any single speed bike should be geared for optimal cadence at the speed where you plan to spend the most time. A track rider would want to spend a small amount of time accelerating to top speed, then spend a relatively long time at top speed. Getting to top speed will be uncomfortable, but the critical test will be maintaining the highest possible speed for the longest period. In stop-go traffic, assuming you slow down to a near stop pretty frequently, you're going to be accelerating a lot more frequently, and spending less time at top speed; so you'll want your bike geared lower. No doubt some compromise is required for either one, of course.