To CLARIFY, my comments about this are NOT about taxis that beep or pull up as a person is walking along (similar to vendors asking you to buy a bracelet etc). In that case I shake my head or a horizontal wave with my hand. The situation I am referring to is when a person is on the road, and WANTS a taxi to stop for them. But a white plate pulls up first, and you know you don't want that. Or a red plate pulls up but something doesn't look or feel right about the situation.