Since 2012, 11 states have legalized marijuana use for adults — which voters nationwide are considering on their ballots this year. Researchers are just beginning to understand the effects of those laws. Colorado and Washington were the first states to legalize the drug, and California, the most populous state in the nation, followed them. Among the most pointed concerns with legalization are whether it has caused more young people to use the drug and whether more people are dying in auto crashes caused by impaired drivers. Data show little change in either area. Surveys of young people in Colorado, for example, show a slight decline in the percentage of middle and high school students using the drug. In Washington, the rates have remained the same.