Get Inside. Stay Inside. Do Not Drive.
The first instinct is to flee. The physics say shelter. Fallout starts landing 10 to 30 minutes after detonation depending on your distance from the blast. A car is the worst possible place to be in: no shielding, maximum exposure, and roads that are about to become impassable anyway. A concrete building, especially a basement or an underground parking garage, reduces your radiation dose by 90 percent or more. It doesn't need to be a bunker. A regular office basement works. The catch is you have to get there in the first ten minutes, which means you needed to know where it was before anything happened.
