What is a Rotary Intersection?
A rotary intersection (or roundabout) is a specialised form of at-grade intersection where all traffic is directed to travel in one direction — clockwise in India — around a central traffic island. Instead of vehicles crossing each other’s paths directly, they weave around the island, converting dangerous crossing and right-turn conflicts into the less hazardous merging and diverging movements.
Key Design Elements of a Rotary
- Entry radius: 15–20 m (urban) | 20–35 m (rural)
- Exit radius: 1.5 to 2.0 times the entry radius
- Central island radius: Generally 1.33 times the entry radius
- Design speed: 30 km/h (urban) | 40 km/h (rural)
- Entry angle: 60° | Exit angle: 30° (entry angle > exit angle)
- Weaving section width: W = (e₁ + e₂)/2 + 3.5 metres
Traffic Operations in a Rotary
- Diverging: Same-direction vehicle separates from main stream upon entry
- Merging: Vehicle from an approach joins the traffic stream circulating in the rotary
- Weaving: Combined movement of both merging and diverging in the same general direction — the primary operation in a rotary
Rotary Capacity Formula (UK Transport Research Lab)
Q = 280W(1 + e/W)(1 − P/3) / (1 + W/l)
Where: Q = Practical capacity of weaving section (PCU/hr), W = Width of weaving section (6m ≤ W ≤ 18m), e = Average entry and exit width = (e₁ + e₂)/2 with constraint 0.4 ≤ e/W ≤ 1.0, l = Length of weaving section with constraint 0.12 ≤ W/l ≤ 0.4, P = Weaving ratio = (b+c)/(a+b+c+d) with constraint 0.4 ≤ P ≤ 1.0.
a and d = non-weaving traffic streams | b and c = weaving traffic streams
Overall rotary capacity = minimum capacity among all weaving sections
Weaving Length — IRC Guidelines
| Design Speed | Minimum Weaving Length | Maximum Weaving Length |
|---|---|---|
| 30 km/h | 30 m | 60 m |
| 40 km/h | 45 m | 90 m |
Maximum weaving length is restricted to prevent over-speeding. Weaving length ≥ 4 × width of weaving section.
Rotary Selection Guidelines
- Suitable when traffic from all approaches is relatively equal
- Volume range: 500 to 3000 PCU/hr
- Very beneficial when right-turn traffic exceeds 30% of total
- Suitable when there are more than four approaches or complex geometry
- Reduces major conflicts (crossing + right-turn) into minor (merging + diverging) only