There is a legal limit for drunk driving, but when it comes to marijuana, new research shows it may be impossible to say just how high is too high to drive. There’s no breathalyzer for pot, and researchers say blood tests are useless when it comes to telling whether someone who has been smoking is fit to drive. The question matters now that four states have made it legal for any adult to smoke marijuana, and more than 20 others have approved its use for. Read the full story