At-grade intersection requirements; rotary design steps

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.