Phoenix, AZ, mid-July, 1976, 11:30 am, shiny silver disk- shaped object hovering over vacant lot at about 200'.

Driving on Interstate 10, downtown Phoenix, on an overpass in the middle of the business district of the city.  My father was driving, my mother in the passenger seat, my niece in the back driver's side, and I in the right back passenger side of the car.
The object was in a position of about 1 o'clock (To my right, SW), hovering at about 200', over a vacant corner lot, on a busy city street, in the midst of tall office buildings.  It's altitude was below some of the tallest buildings, so that it should have been seen by anyone looking in that direction, either from the ground level or out of any of the buildings' windows.  It must have been casting a shadow directly beneath.
It was a highly polished, aluminum-looking disc-shaped craft, round, about the size of a third of a city block (it covered the entire vacant lot).  The best way to describe it would be that it had the appearance and proportions of a fat silver dollar with rounded edges, but absolutely smooth.  There were no extraneous antennas, wings, tail, propellers, insignia, doors, windows, etc..  It hovered motionless, without sound that I could hear (my window was open).
I watched it for about 6 seconds, thinking, "Oh, there's one now."  In other words, I was very calm.  There was no question in my mind that this was an extraterrestrial craft.
I turned my attention to the interior of the car and asked my family, "What's that?!"  Unfortunately, at that exact moment, a small private plane was buzzing the freeway, very low, right in front of us, from right to left (from the direction of the craft).  Everyone in the car said, "It's a plane."  I said, "No, not that!  That!" and as I turned to point to the object, it was gone.  No one else in our car saw it.  I do believe that the people in the small plane came close enough to call it a near miss.  Maybe that was why they were so low over the freeway; they had spotted it and came down for a closer look.
Since it had just disappeared, or had taken off straight up (the only two possibilities, considering the close quarters it was in amongst the tall buildings), I looked up and down in every direction (as much as possible from the back seat of a car) for miles.  The sky was absolutely clear with only one small cloud to obscure any view.  I watched this cloud dissipate, as they do in the desert, trying to see if maybe the object had flown into it.  One can see for miles in the flat desert around Phoenix, and I could not spot it anywhere.
Date of event is approximate.  PD