You can't get any flatter than the Bonneville Salt Flats International Speedway. In late October, the temperature is a very comfortable 50ºF, with no wind and no clouds. The salt looks just exactly like snow, and is firm enough that neither bikes nor trucks leave tracks.
The racetrack itself is exceedingly smooth: the traction is better than snow, but not as good as asphalt. There may be significant drag due to moisture in the salt. A skinny tire bike wouldn't have any trouble riding on the prepared racetrack, but getting any real speed going would be a whole other kind of challenge.