The Department of Transportation may establish such administrative districts as in its opinion shall be necessary for the proper and efficient performance of highway duties. The Department may from time to time change the number of such districts, or it may change the territory embraced within the several districts, when in its opinion it is in the interest of efficiency and economy to make such change. (1931, c. 145, s. 5; 1933, c. 172, s. 17; 1957, c. 65, s. 11; 1973, c. 507, s. 10; 1975, c. 716, s. 7.)