Around where I live, the winner is probably iced coffee containers. Although, because we have a high ratio of prisoners to population - highest in Australia I remember reading - there are plenty of work gangs going around cleaning up the roadsides. Yep, this town is lawless and spotless :-7
In Finland, more on footpaths than on the side of the road, I remember seeing gloves everywhere. Scotland was a bit like that too; I rescued a child's dainty pink mitten from the slush one night and brought it home as a souvenir of my time there.
When I ride my bike on a busy road or highway the main thing I notice on the shoulder are assorted sprockets and grommets that have fallen off passing motor vehicles. You could probably assemble a complete automobile from the parts you see lying by the side of the road.
I do find shoes on the sidewalk quite often.
n.b. hernesheir: I think it has to be soft or grassy to be a verge, though I could be wrong.
I am persistently lobbying through use in daily conversation that we in the US begin to use the lovely noun verge when speaking of the side of the road or thereabouts.
In Florida, I note the following, commonly encountered walking or biking "on the verge": bleached and broken armadillo husks; worthless lottery tickets; cigarette butts; oleaginous (OK, greasy) tufts of the hair of roadkilled opossums; the tiny ziplock bags with the telltale scuffs and stretch marks marking where a small rock of crack cocaine stressed the plastic as it resided in the pocket or fist of a dealer/user.
Shirts, although clingy, have a tendency to break free and unfortunately they pick the worst times and places to do this... like road trips. Perhaps you have noticed the little crosses festooned with buttons along highways. Brings a tear to my eye.
I can't bring myself to talk about shoes right now.