Traffic rotary (Part - 2) - Civil Engineering (CE) PDF Download

Entry, exit and island radius

The radius at the entry depends on various factors like design speed, super-elevation, and coefficient of friction. The entry to the rotary is not straight, but a small curvature is introduced. This will force the driver to reduce the speed. The entry radius of about 20 and 25 meters is ideal for an urban and rural design respectively. The exit radius should be higher than the entry radius and the radius of the rotary island so that the vehicles will discharge from the rotary at a higher rate. A general practice is to keep the exit radius as 1.5 to 2 times the entry radius. However, if pedestrian movement is higher at the exit approach, then the exit radius could be set as same as that of the entry radius.

The radius of the central island is governed by the design speed, and the radius of the entry curve. The radius of the central island, in practice, is given a slightly higher radius so that the movement of the traffic already in the rotary will have priority. The radius of the central island which is about 1.3 times that of the entry curve is adequate for all practical purposes.

Width of the rotary 

The entry width and exit width of the rotary is governed by the traffic entering and leaving the intersection and the width of the approaching road. The width of the carriageway at entry and exit will be lower than the width of the carriageway at the approaches to enable reduction of speed. IRC suggests that a two lane road of 7 m width should be kept as 7 m for urban roads and 6.5 m for rural roads.

Further, a three lane road of 10.5 m is to be reduced to 7 m and 7.5 m respectively for urban and rural roads. The width of the weaving section should be higher than the width at entry and exit. Normally this will be one lane more than the average entry and exit width. Thus weaving width

is given as,

where e1 is the width of the carriageway at the entry and e2 is the carriageway width at exit. Weaving length determines how smoothly the traffic can merge and diverge. It is decided based on many factors such as weaving width, proportion of weaving traffic to the non-weaving traffic etc. This can be best achieved by making the ratio of weaving length to the weaving width very high. A ratio of 4 is the minimum value suggested by IRC. Very large weaving length is also dangerous, as it may encourage over-speeding.

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.

where e is the average entry and exit width, i.e, (e1+e2) 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 32:3 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. Therefore,

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

1. Weaving width at the rotary is in between 6 and 18 meters.

Figure 32:4: Traffic approaching the rotary

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.

Numerical example

The width of a carriage way approaching an intersection is given as 15 m. The entry and exit width at the rotary is 10 m. The traffic approaching the intersection from the four sides is shown in the figure 32:4 below. Find the capacity of the rotary using the given data.

Solution 

• The traffic from the four approaches negotiating through the roundabout is illustrated in figure 32:5.

• Weaving width is calculated as, w = [ e1+e2 2 ] + 3.5 = 13.5 m

• Weaving length, l is calculated as = 4×w = 54 m

• The proportion of weaving traffic to the non-weaving traffic in all the four approaches is found out first.

• It is clear from equation,that the highest proportion of weaving traffic to non-weaving traffic will give the minimum capacity. Let the proportion of weaving traffic to the nonweaving traffic in West-North direction be denoted as pWN , in North-East direction as pNE, in the East-South direction as pES, and finally in the South-West direction as pSW .

• The weaving traffic movements in the East-South direction is shown in figure 32:6. Then using equation,

• Thus the proportion of weaving traffic to non-weaving traffic is highest in the East-South direction.

• Therefore, the capacity of the rotary will be capacity of this weaving section. From equation,