I read the spell to state that it follows its caster.. it'll always be behind the wizard, and not close enough for him to sit upon without it saying "Whoa I'm not behind him!" and then back away in time for him to land on the floor.
This of course, conjured up the alternative of Wizard Mass Transit Systems. One wizard can't do squat, but with 10 lined up, one mage can sit upon the Disk of the man infront of him who cast it... and they can make this line as long as they want. The only catch is that the "Driver" of this magic school bus would either have to walk, or ride a magic carpet. That said, it still allowed for the stately processions of wizards in pointy hats discussing the latest vintage of Githzerai Void Cheese heading down the cobblestone streets without so much as a single bump... while nobles with palanquins pass at a leisurely pace wondering when they'll all grow up and discover mundane labor.