WASHINGTON (AP) — The legalization of recreational marijuana in two states — Colorado and Washington — and medical marijuana in more than 20 others has raised concern that there will be more drivers stoned behind the wheel. Traces of THC, the psychoactive ingredient in marijuana, can be detected in the blood of some habitual marijuana users days or weeks after they last used the drug, making it hard to use blood tests to discern a current level of impairment. A University of Colorado Medical School study found that the share of drivers involved in fatal motor vehicle crashes in Colorado who tested positive for marijuana more than doubled between 1994 and 2011. A National Safety Council study looked at the prevalence of drivers involved in fatal crashes who tested positive for marijuana in 12 states from 1992 to 2009 — before and after implementing medical marijuana laws. The National Highway Traffic Safety Administration is conducting research to get a better idea of how pot affects driving.