California Code of Regulations
Title 13 - Motor Vehicles
Division 2 - Department of the California Highway Patrol
Chapter 2 - Lighting Equipment
Article 11 - Deceleration Signal Lamp Systems
Section 731 - Operating Requirements

(a) Function. The system shall operate so as to indicate a component of deceleration of the vehicle on which it is installed by varying the flashing rate of a yellow lamp when the service brakes are applied.

(b) Reduced Nighttime Brightness. The system shall incorporate an automatic means for reducing the intensity of the lamp during darkness. The system shall cause the voltage to the deceleration lamps to decrease to 5.0 V ± 10% at 0 g deceleration during darkness. The specified voltage shall be reached when the illumination on the sensor is not more than 53.8 lm/m2 (5 lm/ft2) nor less than 5.4 lm/m2 (0.5 lm/ft2).