Design factors of rotary intersection

Design elements/factors for a rotary (IRC:65-1976, Recommended Practice for Traffic Rotaries)

1. Design speed: 30 km/h for urban and 40 km/h for rural rotaries (IRC:65).
2. Radius of the central island and entry/exit curves: the central island radius is about 1.33 times the entry curve radius (IRC:65); the entry curve radius is about 20 m (urban) to 25 m (rural); exit curves are about 1.5 to 2 times the entry curve radius, to speed up exits.
3. Weaving length and width: the weaving length between the entry and exit of consecutive arms should be at least 4 times the weaving width (IRC:65), and is checked so that $w/l$ lies between 0.12 and 0.4 for the capacity formula.
4. Width of the rotary carriageway: the width of the weaving section is the average of the entry width and the non-weaving width plus 3.5 m ($w = (e_1 + e_2)/2 + 3.5$ m); minimum 7 m for a rotary on two-lane approaches.
5. Entry and exit widths: the entry width $e_1$ and non-weaving width $e_2$ are provided to suit the approach carriageways, keeping $e/w$ between 0.4 and 1.0.
6. Shape of the central island: circular (preferred), elliptical, tangent or turbine shapes to suit the angles of the approach roads.
7. Traffic volume and proportion of weaving traffic: the capacity formula depends on this proportion $p$ (0.4-1.0).
8. Super-elevation and cross slope: the carriageway is given a reverse camber of about 1.5 to 2 percent (cross fall outward) for drainage; no superelevation inwards.
9. Sight distance at entries and in the circulating carriageway.
10. Channelising islands, pedestrian refuges, markings, signs and lighting.

Capacity check: the practical capacity of the weaving section (IRC:65, Wardrop/TRRL formula) is

$$Q_p = \frac{280\,w\left(1+\dfrac{e}{w}\right)\left(1-\dfrac{p}{3}\right)}{1+\dfrac{w}{l}}\ \text{PCU/h}$$

where $e = (e_1+e_2)/2$, $w$ = weaving width (m), $l$ = weaving length (m), and $p$ = proportion of weaving traffic to total traffic in the weaving section. The formula is valid when $0.4 \le e/w \le 1.0$, $0.12 \le w/l \le 0.4$, $0.4 \le p \le 1.0$ and $w$ lies between 6 and 18 m.