Not scary but pretty odd: I was in Philadelphia one Saturday morning when they had Martin Luther King Drive closed to traffic - no cars, just bikes and joggers. Really fun to blast along that 4 lane, 4 mile stretch through the trees towards the river. I was rolling along when I heard a car overtaking me, car tires quite clear and loud. I pulled over thinking "that's not right". Then a tightly packed group of about 25 bikes zoomed past - the sum of all those tires sounded like a car. A first for me.