It makes a lot of sense that Line 4 is fully automatic because it runs from some of the richest areas of the city through the largest commercial area (think Manhattan) and into historic downtown. Because of all the people who need to travel between these economically powerful neighborhoods, rush hour on Line 4 means the trains arrive every fifteen seconds. This is only possible because the whole line is controlled by computers and constant moderation. With human train drivers, you would not be able to safely move that many people so quickly.
