When police spotted Freddie Gray and he took off running through his Baltimore neighborhood, officers made a split-second decision to give chase, setting in motion his death in custody and rioting in the streets. Fleeing from police is not, by itself, illegal in America, and the U.S. Supreme Court has made clear that in safe neighborhoods, people not suspected of criminal activity can ignore a police officer who approaches them, even to the point of walking away. But courts have set a different standard for places where street crime is common, ruling that police can chase, stop and frisk people...