Rotary Intersection of Roads and its Design Factors

What is Rotary Intersection of Roads?

The rotary intersection of roads is also called as traffic rotary which is nothing but enlarged intersection of roads where vehicles cross roads or change their direction without stopping.All vehicles coming from different roads move in single direction around the central island and diverges into required exit.

Design Factors for Rotary Intersection of Roads

There are many factors to be considered while designing traffic rotary as follows:

1. Design speed
2. Shape of central island
3. Radius of rotary roadway
4. Weaving angle and weaving distance
5. Width of carriageway at entry and exit
6. Width of rotary roadway
7. Curves at entrance and exit
8. Capacity of rotary
9. Channelizing islands
10. Camber and super elevation
11. Sight distance
12. Lighting
13. Traffic signs
14. Pedestrian ways

Design Speed of Rotary Intersections

At rotary intersections, the vehicle can be moved without any breaking. So, there is no chance to stop the vehicle while other vehicles can change their direction or cross each other without stopping. But, the speed of vehicular movement should be reduced.

Standard highway speed is not applicable at rotary intersection. It should be passed with low speed which will results the safe carrying towards one’s direction without any collision. Speed restriction board should be provided on the road towards rotary intersection.

Shape of Central Island

The shape of Central Island provided for rotary intersection should not contain any corners. It should be formed by curves to allow the comfortable rotations around it.

The shape is particularly dependent upon number of roads meeting at that particular junction. The shapes generally provided are circular, elliptical, turbine and tangential. Circular shape island is shown below:

Radius of Rotary Roadway

The radius of roadway or pavement around the central island is dependent of shape of Central Island. If it is circular shape, radii are similar at all points and if it is elliptical or tangent radii is different at different points. The radius of rotary roadway should be designed by just considering the friction force and super elevation should be neglected.

Normal radius of roadway in curves

But super elevation (e) is neglected i.e., e = 0

Hence, radius of rotary

Where f = coefficient of friction = 0.43 to 0.47

Weaving Angle and Weaving Distance of Rotary Intersections

Weaving angle is the angle formed by paths of vehicle entering the rotary and other vehicle leaving the rotary at adjacent road. The exits of two vehicles may be different but they travel in same way for some short distance in the rotary section which is nothing but merging of vehicles and when the required exit is come two of the vehicles diverged into different directions.

The length of which the two vehicles travel in same way is called as weave length. The weaving angle should be small but minimum of 15o is maintained. The weave length should be at least four times the width of weaving section.

Width of Carriageway at Entry and Exit

The width of carriage way at entrance rand exits is dependent of volume of traffic in that particular region or area. But, the minimum width of 5.0 meters should be maintained for rotary intersections.

Width of Rotary Roadway

Vehicles coming from all directions may meet at a time at least for shorter distance at rotary intersections. The width should be equal to the effective width of weaving section. So, the width provided for rotary roadway should be as follows

Where e1 = width at entrance

e2 = width of non-weaving section

Curves at Entrance and Exit

Entrance and exit curve is nothing but a curve traced by the rear inner wheel of vehicle. Generally, at entrance the vehicle will slow down to design speed of rotary intersection so, at the entrance curve radius can be provided as same as radius of central island.

Coming to exit curves, the vehicle accelerates at exits hence the radius of curve at exit should be greater than the radius of curve at entrance.

Capacity of Rotary Intersections

The capacity of rotary is derived from the below formula and it is mainly dependent upon capacity of individual weaving section.

Where W = width of weaving section

e = average width of entry and width of non-weaving section for the range of e/W

L = weaving length for the range of W/L

Where p = proportion of weaving traffic = (b+c)/(a+b+c+d) = (0.4 < p < 1.0)

a = left turning traffic moving along left extreme lane

b = weaving traffic turning toward right while entering the rotary

c = weaving traffic turning toward left while leaving the rotary

d = right turning traffic moving along right extreme lane

Channelizing Islands

Channelizing islands are provided at entrance or exit of road way to prevent the vehicle from undesirable weaving.

Camber and Super Elevation

We already discussed that the super elevation for rotary roadways is neglected. But, here if the vehicle is changing its direction to its opposite side it will travel around the central island and changes the direction.

While changing, the vehicle may over turn or slip, to overcome this, minimum cross slope is provided which is nothing but camber. This camber acts as super elevation in case of rotary roadways.

Sight Distance

The sight distance provided at rotary intersections should be as higher as possible and in no case the value must be less than the stopping sight distance.

Lighting of Rotary Intersections

The edge of Central Island should be installed with lights which is mandatory. Additional lights may also be provided at the kerbs if the diameter of Central Island is more than 60 m. sometimes, entrance and exit curves can also be provided with lights.

Traffic Signs at Rotary Intersections

Traffic signs should be installed on approaching roads to indicate the presence of rotary intersection ahead to the roadway users. Kerbs at rotary intersections should be coated with black and white strips to improve visibility. Traffic signals should be placed 1 meter above road level to indicate the direction of exit.

Pedestrian Ways at Rotary Intersections

At rotary intersections, the vehicles will move consistently and will not stop. So, the footpath is provided guard rails which will block the entrance of pedestrian into roadway. If crossing of road is important and pedestrian traffic is higher, then construction of subways, over bridges is good solution.

Traffic Rotary and Their Advantages and Disadvantages

Advantages of Traffic Rotary

The advantages of providing traffic rotary are as follows:

1. An orderly and disciplined traffic flow is provided by one way movement.

2. Frequent stopping and starting of vehicles are avoided.

3. Direct conflict is eliminated.

4. All turns can be made with ease.

5. A rotary is specially suited for intersection legs ranging from 4 to 7.

6. For moderate traffic,rotaries are self governing and need no control by police or traffic signals.

7. The capacity of rotary intersection is the highest of all other intersections at grade.

Disadvantages of Traffic Rotary

The disadvantages of providing traffic rotary are as follows:

1. Rotary requires comparatively large of land.So where space is limited and costly,the total cost may be high.

2. Where the pedestrian traffic is large,the rotary by itself can not control  the traffic and needs control by traffic police.If the vehicular traffic has to stop to allow pedestrian to cross,the main purpose of rotary is defected.

3. At the places of mixed traffic ,the design of rotary becomes too elaborate and the operation and control of traffic become complex.

4. Where the angle of intersection of two roads is too acute or when there are more than seven intersecting roads,rotaries are unsuitable.

5. When the distance between the intersections on important highways is less,a rotary becomes troublesome. 

6. At the places of mixed traffic ,there is a tendency to violate the traffic regulation of clockwise movement.

7.Where the traffic volume is very low ,construction of a rotary can not be justified.