Sad, but one child killed by someone in a car who was apparently "high" does not spell out mass danger to society. I believe cell phones are probably attributed to more accidents than marijuana. Although I don't think there are any statistic to prove that, its pretty obvious that marijuana causes less accidents than cell phones, or booze. Hell, if we ban everything because one person got hurt, no one would be allowed to eve walk outside, as everything could be seen as a potential danger.
I am not saying MJ is trouble free, good for everyone, etc...it HAS its problems and some people over use. But when compared to drinking, and many other legal things in society, its ridiculous to spend millions a year fighting marijuana while the others are legal.