Wired has an article on roadcasting, an emerging technology that allows vehicles to stream MP3s to any similarly equipped vehicles within a 30 mile radius. It uses 802.11p technology, a Wi-Fi variant designed for vehicles. Seems a little gimmicky. However, one practical application would require a mic. Record your own audio message to broadcast to other cars around you. Some examples:

• “Going a steady 35 in the left lane ain’t gonna cut it, grandma. Move over.”
• “I’m soooooo druuunk.”
• “Roadhead alert.”