--- title: "Traffic light system of Kathmandu: open or closed loop" subject: "Control Systems" chapter: 1 key: "semester_3/control_systems:traffic-light-justify" source: https://www.bctengineer.com/q/semester_3/control_systems/traffic-light-justify site: BCT Engineering --- # Traffic light system of Kathmandu: open or closed loop Control Systems · Chapter 1: Control System Background - Asked in: 2081 Bhadra, 4 marks - Video: [Traffic Lights Open Loop Control System](https://www.youtube.com/watch?v=8sEicZezHvg) — jflorj14 - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/control_systems/traffic-light-justify - Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1 ## Question 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? ## 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. ## Answer 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. ```text 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the usual traffic light system of Kathmandu considered open loop? A. It uses cameras to count vehicles B. It changes lights by a fixed preset timer without measuring traffic C. It adapts to queue length D. It has a feedback sensor Answer: B. The control action does not depend on the output (queue length). (Key point: Why open loop) 2. When a traffic police officer watches the queues and changes the signal, the system becomes A. Open loop B. A manual closed loop with the officer as sensor and controller C. An unstable system D. A linear system Answer: B. The officer measures the output and corrects the control action, closing the loop manually. (Key point: Manual closed loop) 3. How can the traffic light be made closed loop? A. Make the timer longer B. Install detectors or cameras and feed queue data to an adaptive controller C. Remove the lights D. Use fewer vehicles Answer: B. Sensors measure queue or waiting time, which is fed back to adjust the green time. (Key point: How to make it closed loop) ## Related in Control Systems - [Why Kathmandu traffic lights are open loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atraffic-light-open-loop) — Control Systems, chapter 1 - [Control system block diagram and closed loop advantages](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-cl-advantages) — Control Systems, chapter 1 - [Why closed loop response is faster than open loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Acl-faster-than-ol) — Control Systems, chapter 1, asked 4× - [Closed vs open loop step response analytically](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Acl-vs-ol-step-response) — Control Systems, chapter 1, asked 2× - [Define control system and closed loop block roles](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-bd-role) — Control Systems, chapter 1, asked 5× - [Closed loop control system block diagram explained](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-closed-loop) — Control Systems, chapter 1, asked 4× ## Related in other subjects - [Open loop versus closed loop control](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-vs-closed-loop) — Embedded System, chapter 7, asked 6× - [Steps to design closed loop control system](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-design-steps) — Embedded System, chapter 7 - [Open loop and closed loop V/f inverter-fed drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclosed-loop-vf-inverter-drive) — Utilization of Electrical Energy, chapter 3 - [Closed loop better than open loop (cruise)](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-better-cruise-integral) — Embedded System, chapter 7