Section 34. Except as otherwise provided in the following section, the department, person or local authorities having control of any bridge constructed prior to June first, nineteen hundred and sixty on any road shall maintain notices not less than one hundred feet from each end of such bridge, legible at a distance of fifty feet, stating the maximum weight of vehicle with load which the bridge will safely carry. The said department, person or local authorities may in addition maintain such notices at all other bridges constructed after June first, nineteen hundred and sixty.