Module 5: Traffic Engineering

Lecture 40 Traffic rotaries

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Capacity

The capacity of rotary is determined by the capacity of each weaving section. Transportation road research lab (TRL) proposed the following empirical formula to find the capacity of the weaving section.

$$Q_w = \frac{280 w [1+\frac{e}{w}][1-\frac{p}{3}]}{1+\frac{w}{l}}$$ (1)

where $e$ is the average entry and exit width, i.e, $\frac{(e_1+e_2)}{2}$, $w$ is the weaving width, $l$ is the length of weaving, and $p$ is the proportion of weaving traffic to the non-weaving traffic. Figure 1 shows four types of movements at a weaving section, $a$ and $d$ are the non-weaving traffic and $b$ and $c$ are the weaving traffic.

Figure 1: Weaving operation in a rotary

Therefore,

$$p=\frac{b+c}{a+b+c+d}$$ (2)

This capacity formula is valid only if the following conditions are satisfied.

1. Weaving width at the rotary is in between 6 and 18 metres.
2. The ratio of average width of the carriage way at entry and exit to the weaving width is in the range of 0.4 to 1.
3. The ratio of weaving width to weaving length of the roundabout is in between 0.12 and 0.4.
4. The proportion of weaving traffic to non-weaving traffic in the rotary is in the range of 0.4 and 1.
5. The weaving length available at the intersection is in between 18 and 90 m.