I agree with most of you in general about the roadside memorials. No, they do not make any real logical sense, and no there should not be big religious displays on public property, especially where they pose a hazard.
However, I would not try to stop a bereaved person from putting a small cross in a quiet street corner near where their loved one died. I do not know how old this practice is or what the cultural origins are, but I have noticed it more among the latino community. (At least that is who does it more in my neighborhood.)
What most people do about death is not rational-- of course the dead person's soul or spirit is not lingering at the death site. I myself would be kinda creeped out every time I passed the place if there was a permanent marker where my relative died.
But even cemetaries are not rational-- why use the land that way for a dead person? The rational thing to do is to cremate or give the body for research. I think everyone should do this. But most people's reactions to death are still holdovers from more scary primitive times.
I think people should be allowed to put up their memorials. But the size of the cross or whatever symbol should be regulated to keep it from being a hazard, and there should be a time limit after which it is removed, say a year. Otherwise, the anti-memorial group comes across as the parody of what religious people think of atheists-- people who don't have any empathy at all for the relatives of someone who has died.
Nobody here would say to the grieving parent, "Your kid is not in heaven, he's rotting and being eaten by worms. What a waste. You should have donated his body to the CSI people for research. And hey, he was young, poor and latino, so statistically, he was going to end up dead one way or another. At least the car crash was quick."
I am afraid that people hear the above when we say, "You cannot symbolically mark this spot where your kid died. And BTW, crosses are stupid."
Lots of things are stupid and we need to pick our battles.