Esto no es un simulacro (1962)

Late one night on a rural road in the western US, a policeman sets up a roadblock and begins stopping all traffic. When several cars and a big truck have stopped, he tells everyone why: a nuclear attack on the US appears to be underway, and since they have the truck there, their best chance at survival is to use it as a bomb shelter. Time is short and tensions in the small group quickly rise.