The minimum width of the right-of-way required for the construction, maintenance and safe operation of state highways is hereby fixed at sixty-six feet. But the department, in its discretion, may accept a lesser width than sixty-six feet within incorporated towns or where existing structures of a permanent nature would necessarily be moved or damaged in order to afford the full minimum width of sixty-six feet. And the department may acquire such additional width above the minimum herein fixed as in its judgment may be necessary to meet the exigencies of construction, maintenance, and safe operation of any particular highway.