The assessment of benefits in any viaduct improvement district described in § 14-321-101 shall be made according to the actual benefits the lands will realize from the making and maintenance of the improvement. The commissioners or assessors in the district, in making the assessment of benefits, shall not be required to make them in zones of any form, or proportion them in circular zones of any particular width with reference to the viaduct. However, the commissioners or assessors are not prohibited from making their assessment of benefits on the basis of zones circular or in other form, if, in their opinion, this is the best basis on which to ascertain and fix the amount of the benefits to the several tracts and parcels of land in the district.