The other thing that's particularly gross is that, of necessity, the dirty diapers are filled with shit and/or piss. Much hand-wringing and money and planning and legislation deal with treatment of sewage from our toilets, and yet people can just carelessly toss out contamination bombs (read: dirty diapers) like nothing, with little to no ramifications. It's offensive and disgusting!!
Apparently, at least in my state, there's a 1-800 number you can call to report people who toss cigarette butts out their windows. I would hope that if you called on someone tossing a diaper, the highway patrol would be able to trace the vehicle and slap a fine on moomy/duhdee, as well.