Robert,

Actually, I'd bet that using both long exposure and multiple exposure would settle it, if you know your shutters well enough. I photographed a night scene in DC a couple of months ago, like a two minute exposure. I was standing on a sidewalk and had the problem of cars stopped at a stoplight right at front of me, but not all the time. So I counted seconds and broke up the exposure every time the light turned red and the cars piled in front of me. Worked well. I imagine it would be even easier with people...if the exposure is long enough, you can just close the shutter as soon as someone appears.

dgh

Nice avatar, by the way.