Some UK costs for adding a lane for comparison - https://publications.parliament.uk/pa/cm201617/cmselect/cmtrans/63/6306.htm https://publications.parliament.uk/

Some UK costs for adding a lane for comparison - https://publications.parliament.uk/pa/cm201617/cmselect/cmtrans/63/6306.htm https://publications.parliament.uk/pa/cm201617/cmselect/cmtr... (roughly £4.5-6.5M a mile, probably $7-10.5M in 2015 money?)