"Approximately one in ten fatal car accident victims test positive for which drug?"
Obviously, marijuana since it wouldn't surprise me if one in ten people use it. If it significantly impairs driving then ban driving under the influence of marijuana, but don't continue prohibition. By the website's logic, we should reinstate alcohol prohibition because a third of car accident victims are under the influence of alcohol.
"Which has more cancer-causing chemicals? Marijuana smoke or cigarette smoke?"
I'm not surprised that marijuana smoke does, but it's irrelevant since cigarette smokers inhale more smoke than pot smokers. And even if marijuana's incredibly harmful, you shouldn't go to prison for hurting yourself.
"Does marijuana smoke increase the user's chance of a heart attack?"
Probably complete bull. Or, possibly there's an unrelated factor. I'm making this up out of thin air to illustrate a point, but maybe pot smokers are more likely to be vegetarians, and maybe they have weaker hearts. It's still possible that marijuana causes it, but again, you shouldn't go to prison for hurting yourself.
"Is marijuana addictive?"
Since funding a 1960's Stanford study that showed that marijuana has little adverse effects, the federal government has been reluctant to fund or run on their own accurate marijuana studies. Thus, it's no surprise that federal scientists, whose job depends on marijuana staying illegal, found that pot is very addictive.
However, in my Social Welfare: Propaganda in the Helping Professions class, we learned that around 9% of pot smokers are addicted to it.
"What are the withdrawal symptoms from marijuana addiction?"
Since only 9% of users are addicted, it's likely that most users don't have serious withdrawal symptoms when they quit using marijuana. Plus, the site again references no credible study supporting their claim.