Cal. Veh. Code § 25353

(a) Notwithstanding Sections 25400 and 25950, a bus operated by a publicly owned transit system on regularly scheduled service may be equipped with illuminated signs that include destination signs, route-number signs, run-number signs, public service announcement signs, or a combination thereof, visible from any direction of the vehicle, that emit any light color, other than the color red emitted from forward-facing signs, pursuant to the following conditions:
1. (1) Each illuminated sign shall emit diffused nonglaring light.
2. (2) Each illuminated sign shall be limited in size to a display area of not greater than 720 square inches.
3. (3) Each illuminated sign shall not resemble nor be installed in a position that interferes with the visibility or effectiveness of a required lamp, reflector, or other device upon the vehicle.
4. (4) Each illuminated sign shall display information directly related to public transit service, including, but not limited to, route number, destination description, run number, and public service announcements.
5. (5) The mixing of individually colored light emitting diode elements, including red, is allowed as long as the emitted color formed by the combination of light emitting diode elements is not red.
(b)
1. (1) An illuminated sign may be operated as a dynamic message sign in a paging or streaming mode.
2. (2) The following definitions shall govern the construction of paragraph (1):
  1. (A) “Paging,” meaning character elements or other information presented for a period of time and then disappearing all at once before the same or new elements are presented, is permitted if the display time of each message is between 2.7 and 10 seconds. Blanking times between each message shall be between 0.5 and 25 seconds.
  2. (B) “Streaming,” meaning character elements or other information moving smoothly and continuously across the display, is permitted if the character movement time, from one end of the display to the other, is at least 2.7 seconds, and the movement time of the entire message does not exceed 10 seconds.
(c) A regulation adopted pursuant to this section shall comply with applicable federal law, including, but not limited to, the federal Americans with Disabilities Act of 1990 (42 U.S.C. Sec. 12101 et seq.).