When one or more existing railroads, over or under which it may be advisable, for the protection of travelers, to erect a bridge, crosses a public road or highway which may hereafter be opened, and the erection of such bridge requires more expense than is reasonable that one or two adjoining townships should bear, or more than is reasonable that the township wherein the bridge is to be located should bear, the court of quarter sessions of the county wherein such bridge is to be erected shall, on the representations of the supervisors or on the petition of any of the inhabitants of the township, order a view, in the manner provided for in the case of roads; and if, on the report of the viewers, it shall appear to the court, grand jury and commissioners of the county that such bridge is necessary, and would be too expensive for such township or townships, it shall be entered on record as a county bridge: Providing, That this act shall not apply to existing roads and highways.