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.
