Is it legal to display an LED sign in the back of your car in
California?  The sign would be placed so that it could be clearly seen
by anyone driving behind the car, and would have about 15 characters
of scrolling text on it.  This is the specific sign I'm talking about:
 http://www.teksolutionsllc.com/download/alpha215co.pdf

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Clarification of Question by moskow23-ga on 29 Jul 2004 17:36 PDT

The sign would be inside the rear window, so it is completely inside
the car.  But placed in such a way so that it is visible from the
outside.

Hello moskow-ga,

I spoke with the California Highway patrol, which you can contact at:
California Highway Patrol
P.O. Box 942898
Sacramento, CA 94298-0001
(916) 657-7261

The office I spoke with after seeing a picture of your light, which I
emailed her, stated that it resembles an emergency light and therefore
would be illegal and you would be stopped for it.

I asked her about the state code, which prohibits flashing lights and
lights resembling emergency or traffic control lights, since I
personally didn't feel it fell into either one of those groups, and
she stated as an officer, she would pull me over if she saw me with
it. It seems that if you have this light in your vehicle, the first
offense is a "warning ticket" asking you to remove the light within 24
hours and prove it to the office that issued the ticket.

?Lighting Equipment

40151.   (a) Whenever any lighting equipment or device does not meet
requirements established by the Department of the California Highway
Patrol, the officer making the inspection shall direct the driver to
remove the lighting equipment or device within 24 hours.

(b) Whenever any lighting equipment or device meets requirements
established by the department but because of faulty adjustment or
otherwise fails to comply with this code, the officer making the
inspection shall direct the driver to make it comply with this code
within 48 hours. "

You can read more of the state code here
http://www.dmv.ca.gov/pubs/vctop/vc/vc.htm
http://www.dmv.ca.gov/pubs/vctop/d12/vc25250.htm
http://www.dmv.ca.gov/pubs/vctop/d12/vc25401.htm
http://www.dmv.ca.gov/pubs/vctop/d12/vc24003.htm
http://www.dmv.ca.gov/pubs/vctop/d17/vc40151.htm

State Patrol Website
http://www.chp.ca.gov/html/offices.html

I found this information going a google search for 
California State Patrol
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=california%20state%20partol&btnG=Google+Search

And using the link the commenter left at the bottom of the page.

If this answer requires further explanation, please request
clarification before rating it, and I'll be happy to look into this
further.
Nenna-GA
Google Answers Researcher

Thanks, the researcher not only found the specific legal codes along
with links, but also spoke to a real highway patrol officer.  Great
answer.

You could spend some time with the california 2004 vehicle code
(http://www.dmv.ca.gov/pubs/vctop/vc/vc.htm). Or call ommeone from the
CA DMV at 1 (800) 777-0133

Hello,

I just wanted to thank you for your kind words and rating. I'm glad
you enjoyed the service.

Nenna-GA