the Michigan Department of Transportation (MDOT) like other state highways in Michigan. As a part of these maintenance responsibilities, the department tracks the volume of traffic that uses the roadways under its jurisdiction. These volumes are expressed using a metric called annual average daily traffic, which is a statistical calculation of the average daily number of vehicles on a segment of roadway. MDOT's surveys in 2010 showed that the highest traffic levels along M-39 were the 159,400 vehicles daily between Schoolcraft Road and Grand River Avenue in Detroit; the lowest counts were the 20,400 vehicles per day between the I-94 and Van Born Road interchanges. All of M-39, and Southfield Road on either end that is not under state maintenance, has been listed on the National Highway System, a network of roads important to the country's economy, defense, and mobility. History Previous designations Two previous highways bore the M-39 designation. When the state highway system was first signed in 1919, the number was assigned to a highway that ran between Grand Rapids and Lansing. The westernmost end in Kent County near Alto was replaced by a realigned M-16. Within the next few years, the western end was scaled back further to the Woodbury area when M-50 was extended, with the remainder replaced by an extended M-43 around 1938. The next trunkline to be designated M-39 was Schaefer Highway in 1939, running north–south from US 25/M-17 (at the five-way intersection of Schaefer, Oakwood Boulevard, and Francis Street) in Melvindale to US 16 (Grand River Avenue) in western Detroit. Since M-39 was moved to Southfield Road in the end of the 1950s, Schaefer Highway has been a locally maintained road. Current designation M-39 was reassigned to Southfield Road, which parallels Schaefer Highway to the west, connecting I-94 with Northwestern Highway in 1958-59. By 1961, the freeway was marked as under construction on maps. The first section opened in December 1961 was from Ford Road north to Chicago Road. The remainder of the freeway between I-94 and the Lodge Freeway and 9 Mile Road was opened by the middle of 1964 at a total cost of $40 million. The highway designation was extended southeasterly along Southfield Road to the new I-75 freeway and a terminus at M-85 (Fort Street) in 1966. In 1987, the M-39 designation was truncated slightly so that the trunkline no longer officially connects to M-85. Exit list See also