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Traffic light system of Kathmandu: open or closed loop

Short answer

The typical Kathmandu traffic light system is open-loop; it operates on a fixed timer without sensing actual traffic, so the lights do not adapt to vehicle queues. To convert it to closed-loop, install sensors like loop detectors or cameras to measure queue length and feed it back to an adaptive controller that adjusts green times, thus reducing waiting time and congestion.

How would you justify that the traffic light system of KTM is open loop control? If you think it is closed loop, how? If not, how would it be closed loop?

The usual traffic-light system of Kathmandu is an open-loop control system, because the lights change according to a fixed time sequence set in a timer. The controller does not measure the actual traffic (queue length, waiting vehicles) and does not change its action according to it.

 timer     +------------+  lights  +---------+ vehicle flow
 setting ->| Timer/     |--------->| Traffic |------------>
 (fixed)   | controller |          | (plant) |
           +------------+          +---------+

Why it is open loop

  • Input: preset timing (e.g. 60 s green, 5 s amber).
  • Output: flow of vehicles / queue length.
  • There is no sensor feeding the traffic condition back to the controller, so the output has no effect on the control action. Even when a road is empty, it still gets its fixed green time.

(When a traffic police officer watches the queues and changes the signal manually, the system becomes a manual closed loop, with the officer acting as sensor and controller.)

How to make it closed loop

Install sensors such as inductive loop detectors, cameras with image processing, or radar at each approach. The measured queue length or waiting time is fed back to an adaptive controller, which compares it with the desired value and lengthens or shortens the green time.

 desired     +      +------------+   +--------+   +---------+  queue
 queue  ------>(Σ)->|  Adaptive  |-->| Lights |-->| Traffic |--+---->
 or delay       ^ - | controller |   +--------+   +---------+  |
                |   +------------+                             |
                +--------[Loop detector / camera]<-------------+

This reduces waiting time and congestion and adapts automatically to peak hours, festivals or accidents.

✓ Answer checked by Claude Opus 5.5 (high effort)

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