The commanding officer of a United States military reservation, for a highway or part of a highway in the military reservation, including a highway of the state highway system, has the same authority by order to alter prima facie speed limits from the results of an engineering and traffic investigation as the Texas Transportation Commission for an officially designated or marked highway of the state highway system. A commanding officer may not modify the rule established by § 545.351(a) or establish a speed limit of more than 75 miles per hour.