Cal. Sts. & High. Code § 101.14

(a) Prior to June 30, 2016, and subject to federal approval as provided in subdivision (d), the department shall update its internal policies to allow displays of the following types of messages on changeable message signs:
1. (1) Safety messages.
2. (2) Transportation-related messages.
3. (3) Reminders to register to vote, as requested by the Secretary of State, not more than two days prior to, and on the last day to, register to vote in a particular statewide general election, statewide primary election, or statewide special election conducted pursuant to the Elections Code.
4. (4) Reminders to vote, as requested by the Secretary of State, as elections approach, not more than two days prior to, and on election day of, a particular statewide general election, statewide primary election, or statewide special election conducted pursuant to the Elections Code.
(b) For purposes of this section, “changeable message sign” means any electronic sign on a roadway with a changeable message typically used to alert motorists of traffic conditions, unusual weather conditions, emergencies, or other events.
(c) Nothing in this section shall be construed to alter the requirements of the Emergency Alert System, the Amber Plan under Section 8594 of the Government Code, or the Blue Alert System under Section 8594.5 of the Government Code. The department may give priority to other messages, including, but not limited to, the display of emergency alerts, Amber Alerts, Blue Alerts, safety messages, and transportation-related messages, over the messages authorized pursuant to paragraph (3) or (4) of subdivision (a).
(d) The department shall not display any information on a changeable message sign authorized pursuant to paragraph (3) or (4) of subdivision (a) unless the United States Department of Transportation, or any of its agencies, has expressly approved the display of that information. If the department is advised by the United States Department of Transportation, or any of its agencies, that the display of information otherwise authorized by paragraph (3) or (4) of subdivision (a) will result in the reduction of federal aid highway funds to the state pursuant to Section 131 of Title 23 of the United States Code, that display of information shall not be made.