The commissioner may construct or reconstruct any bridge on any highway maintained by him when, in his opinion, public convenience and necessity so require. Any bridge constructed or reconstructed on any such highway with two or more lanes shall have a clear width of roadway of not less than twenty-eight feet exclusive of the width of any sidewalk, unless in the judgment of the commissioner a lesser width is warranted. The provisions of this section shall not apply to the construction or reconstruction of any bridge on any highway maintained by a municipality.