I made a post about this a while ago, well a cycling group in Southern California has taken the initiative to get a petition rolling.
Sign the petition communicating with our state politicians and Department of Motor Vehicles to create a "Share the Road" license plate frame in California, such as the one that has been proposed in the state of Colorado. Shown at: http://www.bicyclecolorado.org/
Sign the petition here:
http://www.petitiononline.com/str1234/petition.html
