Why are there so many single shoes on the side of the road or highway?

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Request for Question Clarification by justaskscott-ga on 11 Oct 2006 22:23 PDT

Is this page a sufficient answer?

"Why do you always see just one shoe by the side of the road?" (29-May-1992)
The Straight Dope
http://www.straightdope.com/classics/a3_124b.html

If so, please let me know; I'd be happy to post it as an answer.

I don't have an answer to this question, but since you ask, you might
find it amusing that there's a tree along U.S. Highway 131, in the
Michigan Southern Peninsula, which is festooned with dozens of old,
disgarded shoes. They're all hung from branches by their laces.

Any pair of shoes will be found and picked up by poor wayfarers.

Very few people put both their feet out of the car window.  
(Statistics collected by groups in California that participate in
cleaning up highway shoulders show that 87.367% of the shoes are for
the right foot and 11.37% for the wrong foot.  The difference is shoes
too mangled to be identified as right or wrong.)

There are several components to this. The exact answer will depend on
what probability you feel like assigning to each part.
First: sometimes a single shoe will fall from a vehicle.
Second: both shoes may fall. If this happens at a long interval they
may appear as two single shoes. Or, both shoes fall at the same time,
but one bounces of the road, making it seem a single shoe fell.
Third: when two shoes are on the road then sometimes (as myorin
suggests) they may get picked up. If not, then passing cars will
sometimes strike one or both. Each strike may leave both on the road,
both off the road, or one on and one off. If this is random, then the
distribution of the three states is a binomial function. Over time the
shoes will all end up off the road but before that happens the odds
greatly favor a single shoe being left for part of the time.
So, to see a pair would mean they must have fallen off at the same
time, bounced in a way that both stay close together, not get picked
up by a shoeless passer-by and subsequent hits always leave both on
the road and close to each other.