Maybe I'm wrong, but it sounds like you're wanting a shutter speed that will allow the people to walk by and blur out of existance. You'll still have that residual streak there though. Plus, you'd have to be on a tripod, and with an exposure that long in daylight, the rest of the photo would be over exposed.
I'm not sure that you can elminate the moving people just with shutter speed.
However, I am low on sleep at the moment and maybe I've forgotten something. I hope someone else can lend an idea to help you with what you're wanting to do.
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