Chapter 4. Presumptions of California Vehicle Code >> Division 17. >> Chapter 4.

In any action involving the question of unlawful speed of a vehicle upon a highway which has been signposted with speed restriction signs of a type complying with the requirements of this code, it shall be presumed that existing facts authorize the erection of the signs and that the prima facie speed limit on the highway is the limit stated on the signs. This presumption may be rebutted.

(a) Whenever a traffic sign or traffic control device is placed in a position approximately conforming to the requirements of this code, it shall be presumed to have been placed by the official act or direction of lawful authority, unless the contrary is established by competent evidence.
  (b) Any sign or traffic control device placed pursuant to this code and purporting to conform to the lawful requirements pertaining to it shall be presumed to comply with the requirements of this code unless the contrary is established by competent evidence.

In any case, involving an accident or otherwise, where any rear component of a train of vehicles fails to follow substantially in the path of the towing vehicle while moving upon a highway, the vehicle shall be presumed to have been operated in violation of Section 21711.