---
title: "At-grade intersection requirements; rotary design steps"
subject: "Transportation Engineering II"
chapter: 1
key: "semester_7/transportation_engineering_ii:intersection-requirements-rotary-design-steps"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/intersection-requirements-rotary-design-steps
site: BCT Engineering
---

# At-grade intersection requirements; rotary design steps

Transportation Engineering II · Chapter 1: Traffic Engineering

- Asked in: 2075 Chaitra, 8 marks
- Video: [Lec-21 Guidelines for Rotary Intersection 1| Traffic Engineering and Management | Civil Engineering](https://www.youtube.com/watch?v=Mco1FgsYkac) — Civil Engineering Department_LJIET
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/intersection-requirements-rotary-design-steps
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-1

## Question

What are the basic requirements of intersection at grade? Write down the design steps of rotary intersection.

## Short answer

Key intersection requirements: safety, capacity, simple layout, good sight distance, channelisation, suitable geometry, control devices, and economy. Rotary design steps: collect peak‑hour PCU volumes, fix design speed (30‑40 km/h), select entry radius and weaving width/length (l≥4w), compute weaving proportion p, verify capacity Q_p=280w(1+e/w)(1-p/3)/(1+w/l), and detail islands/markings.

## Answer

#### Basic requirements of an intersection at grade
1. **Safety first:** reduce the number and severity of conflict points; give drivers clear sight of each other (sight triangle, IRC:66 and IRC:SP:41).
2. **Adequate capacity:** the junction must carry the design hour volume of all movements without long delays.
3. **Simple, clear layout:** drivers should understand the movements at once; avoid complex or multiple-road junctions; avoid skew (the angle between roads should be about 90 degrees, not less than 60 degrees).
4. **Minimum conflict area:** keep the paved area small and channelise movements with islands, so vehicles cannot wander.
5. **Good visibility:** approach sight distance and sight triangles should be free of obstructions.
6. **Separation of conflicts** by space (islands, channelisation), by time (signals) or by level (grade separation).
7. **Suitable geometry:** correct approach gradients (flat, not more than about 4 percent), corner radii for the design vehicle, adequate turning lanes and pedestrian facilities.
8. **Proper traffic control devices:** signs, markings, signals or a rotary where needed.
9. **Pedestrians and cyclists** need crossings, refuges and signal time.
10. **Economy:** the cost of land, construction and operation should match the benefit.
11. **Drainage and lighting** for safe all-weather and night use.

#### Design steps of a rotary (IRC:65)
1. **Collect data:** peak-hour volume of each movement (PCU), traffic composition, approach speeds and land available; check that a rotary suits the junction (4 or more arms, entering volume between about 500 and 3000 PCU/h).
2. **Fix the design speed:** 30 km/h (urban) or 40 km/h (rural).
3. **Choose the entry curve radius $R_1$** (about 20 m urban, 25 m rural), the exit radius $R_2 = 1.5$-$2R_1$, and the central island radius $R_c \approx 1.33R_1$.
4. **Fix widths:** entry width $e_1$ and non-weaving width $e_2$ from the approach carriageways; weaving width $w = (e_1+e_2)/2 + 3.5$ m.
5. **Fix the weaving length $l$:** at least 4 times $w$ (usually 30-90 m); the length is measured between the ends of the splitter islands of successive arms.
6. **Compute the weaving traffic proportion $p$** from the turning volumes: $p = (b + c)/(a+b+c+d)$ (the weaving streams over the total traffic in the section).
7. **Check the capacity** of the weaving section:

$$
Q_p = \frac{280\,w(1+e/w)(1-p/3)}{1+w/l}
$$

and confirm $Q_p \ge$ the actual weaving-section traffic; if not, increase $w$ or $l$ and repeat. Check the limits for $e/w$, $w/l$ and $p$.
8. **Design the details:** entry and exit curves, splitter islands, cross fall (outward 1.5-2 %), sight distance at entries, pedestrian crossings, signs, markings and lighting.

## Check yourself

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

1. What should the angle between roads at an at-grade intersection be?
   A. About 90 degrees, not less than 60 degrees
   B. About 30 degrees
   C. Any angle
   D. Not more than 45 degrees
   Answer: A. Skew should be avoided; roads should meet near 90 degrees and not less than 60 degrees. (Key point: Skew angle)

2. In what three ways can conflicts be separated?
   A. By speed, colour and sign
   B. By space, time and level
   C. By lane, kerb and shoulder
   D. By cost, land and law
   Answer: B. Space (islands), time (signals) and level (grade separation). (Key point: Separation of conflicts)

3. In the rotary design steps, the central island radius is about:
   A. 0.5 R1
   B. 1.33 R1
   C. 3 R1
   D. Equal to weaving length
   Answer: B. Rc is about 1.33 times the entry radius R1; exit radius is 1.5 to 2 R1. (Key point: Rotary radii)

4. If the weaving section capacity Qp is less than the actual weaving traffic, what is done?
   A. Accept the design
   B. Increase w or l and repeat the check
   C. Reduce the entry curve radius
   D. Remove the splitter islands
   Answer: B. Increase weaving width or length and recompute until Qp meets the weaving traffic. (Key point: Capacity check action)

## Related in Transportation Engineering II

- [At-grade intersection requirements and night visibility](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aintersection-requirements-night-visibility-lighting) — Transportation Engineering II, chapter 1
- [At-grade intersection requirements; street lighting importance](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aintersection-requirements-street-lighting-importance) — Transportation Engineering II, chapter 1
- [At-grade requirements; grade separated intersection](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aintersection-requirements-grade-separated) — Transportation Engineering II, chapter 1
- [Design factors of rotary intersection](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Arotary-design-factors) — Transportation Engineering II, chapter 1
- [Traffic capacity types; rotary intersection sketch](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atraffic-capacity-types-rotary-sketch) — Transportation Engineering II, chapter 1
- [Rotary intersection: advantages and disadvantages](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Arotary-definition-advantages-disadvantages) — Transportation Engineering II, chapter 1

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