I think the same thing about people that release balloons in the air. This used to be really popular thing to do, I don't know so much anymore, but I hate it.
I think roadside memorials are a nice way to remember a loved one, a part of the healing process for some and also serve as a reminder to others on the road who see it-to take more care when driving. However, I do think that ordinances should be passed that mandate memorials to be removed after a number of days or maintained-whatever. They should also be constructed of natural or biodegradable materials. Fresh flowers, pinecones, sea shells, ect. That would help.