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:
Shape of central island
Radius of rotary roadway
Weaving angle and weaving distance
Width of carriageway at entry and exit
Width of rotary roadway
Curves at entrance and exit
Capacity of rotary
Camber and super elevation
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 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.
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.