In a situation like Houston they should probably just close the freeway to low occupancy vehicles. 4 people per car minimum, and have free evac buses picking people up at all points in the region. In fact the contraflow lanes could be reserved for buses and emergency vehicles, no cars allowed. Use TV and radio broadcasts to urge people to evac by bus instead of car, and hire some big name rock bands to put on free shows at the evac centers that can only be reached by bus. That ought to do a lot towards lowering traffic congestion in an evac.