Driving south on S. 56th St. toward Pine Lake Rd. Time ~ 7:15am. on Sun., 9-1-13. Cloudy day...there had been thunderstorms to the south of us and and lite and dark gray clouds persisted as they moved in a SE direction. No rain at the time of the sighting tho the pavement showed there must have been a brief shower. No lightning/thunder at the time I was there. Breeze, but not windy. Comfortable temperature. As I drove up the upgrade from the RR crossing, I was the only vehicle going either way...very quiet traffic day. I was ~ a bit N of the Shadow Pines apt. complex when I noted the larger white rectangular box-like 'thing' crossing 56th St. from east to west. I noted I had not seen anything approaching and no vehicle like a truck pulling it, and then realized there were no wheels. It appeared to move straight across the street about roof height, but not necessarily over the rooftops. Maybe going about 20-30 mph. If it had turned off 56th St., straightened out and was being towed, I don't think it could have gone that fast that quickly. Behind it was the horizon with the dark gray clouds. It seemed to just disappear. I was going about 45 mph, the speed limit, and in the southbound lane, which widens at the point where Shadow Pines Ln is, at which time I moved to the curb lane (southbound). As I passed Madalyn Rd, I looked down it to the west and saw nothing...just houses, a few cars parked on the street. No trucks, no trailers, no activity at all. I didn't note any trailers parked anywhere. The street curves some, but I was only a few seconds behind and could see quite a ways down, and saw nothing like what I had seen crossing the roadway.Overall, it seemed, like I noted, about the size of a popup camper when everything is packed and stowed. Except there were no other vehicles. My window was down halfway and the radio was quite low with blues music, but I heard nothing unusual, as engines, or anything. I was surprised to see nothing down the side street. It made me feel, "Am I losing my mind?" "What was that?!" "Is this how Alzheimer's (or other dementia) starts?" I have always believed we could have 'visitors', because 'we' are indeed here. But this is the first I recall an 'event' such as this. I still don't know any more about it, and would like to. I am willing to accept a reasonable explanation for what I saw, but no one has had one at all, let alone one that makes sense. I thought and thought about whether I'd missed something. It wasn't a 'flash' of a 'feeling' or a 'sight'...It was definitely moving directly across the path I was on...and then gone when I approached just a few secs. later.