(a) No person shall drive a vehicle over any bridge or other elevated structure constituting a part of a highway at a speed which is greater than the maximum speed which can be maintained with safety to such bridge or structure, when such structure is posted as provided in this Code section.

(b) The Department of Transportation may conduct an investigation of any bridge or other elevated structure constituting a part of a highway, and, if it shall thereupon find that such structure cannot, with safety to itself, withstand vehicles traveling at the speed otherwise permissible under this chapter, the department shall determine and declare the maximum speed of vehicles which such structure can safely withstand and shall cause or permit suitable signs stating such maximum speed to be erected and maintained before each end of such structure.

(c) Upon the trial of any person charged with a violation of subsection (a) of this Code section, proof of determination of the maximum speed by the department and the existence of such signs shall constitute conclusive evidence of the maximum speed which can be maintained with safety to such bridge or structure.