Much like Stapfam and others have said a lot of the debris left on the road have nothing to do personally with cyclists it is either thoughtless litter or thoughtless youth. Glass and other things tossed on city basketball courts, tennis, even open handball courts aren't directed and people playing basketball, tennis or handball they are directed at society in general. They are parts of an act of defiance saying, "we don't care and that is just who we are." I have seen broken glass in places where no one cycles, tacks on running paths, Trash cans knocked over on hiking trails and I imagine the same type of people are responsible for all of it. We just have to do as other cyclists, hikers, runners, walkers and even drivers have to do when the same kind of people toss roofing nails on roadways, realize these problems are a minority of the things we deal with and keep doing we enjoy.