The
challenging part for government is creating a public warning
system inexpensive enough, its available to everyone, not just
people who can afford private airplanes.
We could use the one that was already built for this: The NOAA All
Hazards radio network (http://www.nws.noaa.gov/nwr/). It uses 7
nationwide frequencies and 1025 radio transmitters to cover 95% of
the US.
It costs around $20 for a basic receiver. A car could easily just
listen to the 7 frequencies and wait for the alert tones. If you
want to get fancy you could overlay GPS to detect the county and use
that to filter the alerts to the county you're currently in.