Every railroad corporation shall cause signs to be placed and constantly maintained alongside of each public road or street where it is crossed by the railroad on the same level. Each such sign shall be elevated so as to be easily seen by travelers and on each side of it shall be printed in large letters the words "Railroad Crossing" unless the railroad corporation elects to place two of such signs at each crossing, one on each side of the track in which case each of such signs may have thereon in large letters, only on the side facing the traffic approaching the crossing, the words "Railroad Crossing." But this section shall not apply to streets in cities, towns and villages unless the corporation be required to put up such sign by the officers having charge of such streets.