The state highway commission upon request from any local authority shall, or upon its own initiative 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 article, the commission shall determine and declare the maximum speed of vehicles which such structure can withstand, and shall cause or permit suitable signs stating such maximum speed to be erected and maintained at a distance of one hundred feet before each end of such structure.
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 signposted as provided in this section.
Upon the trial of any person, charged with a violation of this section, proof of said determination of the maximum speed by said commission and the existence of said signs shall constitute conclusive evidence of the maximum speed which can be maintained with safety to such bridge or structure.