It looks to me like some of the geometry (roads, sidewalks) are being rendered in LODs, which is what you usually see far away down the road, or if you're flying far above in a helicopter. The LOD is never representative of actual collisions and may cause it to look like objects, vehicles are floating.
Are you absolutely sure you don't have command lines or mods in that would force the game to run at lower graphics quality? Or maybe your system is running low on memory. Those are really the only things I can think of that would cause that.