Angeline Chilton says she can't drive unless she smokes pot. The suburban Denver woman says she'd never get behind the wheel right after smoking, but she does use medical marijuana twice a day to ease tremors caused by multiple sclerosis that previously left her homebound. "I don't drink and drive, and I don't smoke and drive," she said. "But my body is completely saturated with THC." Her case underscores a problem that no one's sure how to solve: How do you tell if someone is too stoned to drive?