---
title: "Why Kathmandu traffic lights are open loop"
subject: "Control Systems"
chapter: 1
key: "semester_3/control_systems:traffic-light-open-loop"
source: https://www.bctengineer.com/q/semester_3/control_systems/traffic-light-open-loop
site: BCT Engineering
---

# Why Kathmandu traffic lights are open loop

Control Systems · Chapter 1: Control System Background

- Asked in: 2078 Kartik, 4 marks
- 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-open-loop
- Whole chapter: https://www.bctengineer.com/md/semester_3/control_systems/chapter-1

## Question

Why do you think traffic light system in Kathmandu is an open loop control system?

## Short answer

The traffic light system in Kathmandu is open loop because it operates on fixed timers without sensors measuring actual traffic flow. There is no feedback of output (queue length) to adjust the light timing, so it cannot correct for disturbances.

## Answer

The traffic-light system in Kathmandu is **open loop** because its control action (which light is on and for how long) **does not depend on the output** (the actual traffic flow or queue length). The lights change according to a timer that is set in advance.

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

#### Reasons
1. **No feedback sensor:** there are no vehicle detectors (loops, cameras) feeding traffic data to the controller.
2. **Fixed time sequence:** green, amber and red times are preset; an empty road gets the same green time as a crowded one.
3. **No comparison with a desired value:** the controller has no error signal; it cannot know whether the queue is growing.
4. **Disturbances are not corrected:** a sudden rush, accident or VIP movement is not handled automatically. In practice traffic police switch to manual control, which itself shows the automatic system has no feedback.

To make it closed loop, sensors would measure queue length or waiting time, and an adaptive controller would adjust the green time accordingly.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is the Kathmandu traffic light open loop?
   A. Its control action does not depend on the actual traffic flow
   B. It uses feedback
   C. It has vehicle detectors
   D. It compares queue to desired value
   Answer: A. Lights follow a timer set in advance. (Key point: Reason for open loop)

2. In an open-loop traffic light, an empty road gets
   A. No green
   B. The same green time as a crowded one
   C. Longer green
   D. Shorter green
   Answer: B. Times are preset and not adjusted. (Key point: Fixed sequence)

3. Why does the controller lack an error signal?
   A. No comparison with a desired value is made
   B. The error is infinite
   C. Sensors are too fast
   D. Lights are too dim
   Answer: A. Without feedback there is nothing to compare with. (Key point: Missing error signal)

4. To make it closed loop, the controller should adjust
   A. Lamp colour
   B. Green time according to measured queue or waiting time
   C. The timer's brand
   D. The road width
   Answer: B. Sensors feed queue data to an adaptive controller. (Key point: Closing the loop)

## Related in Control Systems

- [Traffic light system of Kathmandu: open or closed loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atraffic-light-justify) — Control Systems, chapter 1
- [Control type of hydropower governor and traffic lights](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Ahydro-governor-traffic-type) — 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

## 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×
- [Open-loop cruise control using P control](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-loop-cruise-p-control) — Embedded System, chapter 7