If you come upon a severe dust storm, you should:

If you encounter a severe dust storm, you should immediately check traffic around your vehicle, begin slowing down, drive completely off the highway and stop as far to the right as possible (taking care not to stop in the travel lane or emergency lane), turn off your lights, take your foot off the brake, stay in the vehicle with your seat belt(s) buckled, and wait for the dust storm to pass.
DMV Writen Test