The commissioners in assessing the benefits to lands, public highways, railroads and other rights of way not traversed by such works and improvements as provided for in the Plan of Reclamation shall not consider what benefits will be derived by such property after other ditches, improvements or other plans for reclamation shall have been constructed but they shall assess only such benefits as will be derived from the construction of the works and improvements set out in the Plan of Reclamation or as the same may afford an outlet for drainage or protection from overflow of such property. The commissioners shall give due consideration and credit to any other canal, drain, ditch, dyke, levee or other system of reclamation which may have already been constructed and which affords partial or complete protection to any tract or parcel of land in the new district. The public highways, railroads and other rights of way shall be assessed according to the increased physical efficiency and decreased maintenance cost of roadways by reason of the protection to be derived from the proposed works and improvements.