In general I hate when cars try to wave me by (as a pedestrian) w/ little regard to the total situation. I realize they're being nice but often all they're doing is making a bigger mess for the bigger picture. I always try to wave them along instead, but sometimes they're insistent, and I can almost hear them grumbling "why won't this dummy go?" ... sorry, it's because you're a) holding up traffic behind you and b) I couldn't anyways as there is other traffic I'd need to cross.