Across swaths of Florida, South Carolina, Georgia, and North Carolina, half the highway lanes have reversed. Traffic engineers call this "contraflow," the volte-face of normal traffic. Now, on both sides of these roads, vehicles only run one way-away from Hurricane Matthew. As the Category 4 storm ripped the Caribbean with 145 mph winds, local and state officials urged nearly two million Americans to flee their homes before it hit the southern Atlantic coast.