6.6. Several complaints recently have been sent to the Gotham City police department regarding the increasing incidence of congestion on the city's streets. The com- plaints attribute the cause of these traffic tie-ups to a lack of synchronization of the traffic lights. The lights are controlled by a main computer system, and ad- justing this program is costly. Therefore, the controllers are reluctant to change the situation unless a clear need is shown. During the past year, the police department has collected data at 1,000 inter- sections. The data were compiled on a monthly basis as shown below: