TRAFFIC JAM MONITORING SYSTEM

PROBLEM TO BE SOLVED: To provide a traffic jam monitoring system which can accurately decide the state of a traffic jam that is caused at a road construction site. SOLUTION: This system includes the signals placed at every vehicle entrance of an alternate pass section where plural paths entering a single lane are prepared on a road and the vehicles traveling on the lane in different directions pass alternately in each prescribed permitted time, a vehicle detector which detects presence or absence of vehicle passing through the alternate pass section and a controller which controls the permitted entering directions of vehicles in the alternate pass section by switching the signal display, decides whether the vehicles traveling in the permitted entering directions that are detected by the vehicle detector passing the alternate pass section pass or not continuously in the permitted pass time and then decides occurrence of a traffic jam when the continuously passing events are caused in the same direction with each passing permission.