Why is there always only one?

I am certain there are plenty of you out there who have noticed the odd shoe, or glove, by the side of the road, be it a motorway, A-road or some little one-track lane that takes you on a meandering (and sometimes  nail-biting, white-knuckle) trip down some long ago-tarmacadammed ancient road.  I am equally certain that of those of you who have noticed,  there a quite a few who have wondered...why just one?  For this rather odd exercise I am going to leave out the question of gloves.  In most cases these have been those large, rubber things that workmen wear so in some respects a single glove may be explained - road workers leaving them behind for example.  

I shall concentrate on footwear, but must emphasise that they are not always shoes; they are sometimes walking boots, Wellingtons, trainers, or sandals (I have not yet seen slippers I must admit, but probably just as well as that would really throw a whole new spanner into the works). 

And are they all left shoes?  I have never done an in-depth survey on such things.  It would make sense in a way if one takes into account that they could have been tossed from the passenger side of the vehicle - presuming that said owner was in the passenger seat.  I can only hope that the driver would not contemplate removing a shoe and tossing it out of the window in a wide lob over the roof, or perhaps passing it to someone in the passenger seat, or even just tossing it out of the passenger door window while driving at high speed down the motorway.

Or are these items of clothing part of some initiation exercise?  Will they become part of some future urban myth?

Perhaps the owners were abductees of some UFO that sent down a tractor beam and sucked them up into the bowels of the mothership?  But why leave one shoe?   An untied lace, thus rendering the footwear a tad loose, and - hence - maybe resulting in it escaping from its owner's abduction?    Or perhaps they were from cases of spontaneous combustion - nah that can't be it.  People don't spontaneously combust on motorway verges as a regular rule.

Or is it simply that one person accidently (but how?) did it, then someone else copied just because they could, and over the years since then others have perpetuated the nonsense just for the sake of making everyone else think something weird is going on?