The most closely controlled study of it's kind now finds that while alcohol in a person's system increases risk of accident by four times or more, THC in a person's system increases risk of accident by only 5%. A Washington Police Commander states that drivers found under the influence of marijuana are not typically making road mistakes, but are pulled over for other traffic violations and found to be on pot after the fact.
“Marijuana is complicated. This study is a very important part of beginning to learn the factors we need to pay attention to,” Trowbrigde says. “We know far less about marijuana at this point than we do about alcohol.”