Six members of the county road administration board shall be county legislative authority members and three members shall be county engineers. If any member, during the term for which he or she is appointed, ceases to be either a member of a county legislative authority or a county engineer, as the case may be, his or her membership on the county road administration board is likewise terminated. Three members of the board shall be from counties with a population of one hundred fifty thousand or more. Four members shall be from counties with a population of from thirty thousand to less than one hundred fifty thousand. Two members shall be from counties with a population of less than thirty thousand. Not more than one member of the board shall be from any one county.