Rotary Intersection in traffic engineering.pptx

Very Important design

1. Rotary Intersection (Roundabout) A rotary intersection is an enlarged road intersection where all converging vehicles are forced to move round a large central island in one direction (clock wise direction). In a rotary, the crossing of vehicle prevented by allowing all the vehicles to merge into the traffic streams around the rotary and then to diverge out to the desired radiating road. Crossing conflicts are eliminated and converted into weaving aspects. Weaving is the combined movement of a merging and diverging of traffic streams moving in the same streams. Weaving g length is defined as the length of a section of a rotary in which weaving occurs

2. Diverging : The dividing of a single stream of traffic into separate streams. Merging : The converging of separate streams of traffic into a signal stream. Weaving : The combined movement of merging and diverging of traffic streams moving in the same general direction. Weaving length : The length of a section of a rotary in which weaving occurs.

3. Advantages of Rotary Intersections The main objective of providing a rotary are eliminate the necessity of stopping even for cross streams of vehicles and to reduce the area of conflict. All traffic including those turning right or going straight across the rotary have equal opportunity as those turning left. An orderly & regimented traffic flow is provided by rotary one-way movement. Normally , all traffic proceeds simultaneously & continuously at fairly uniform, though low speed. Frequent stopping & starting are avoided. All turns can be made with ease, although little extra travel distance is required for all movements except left turns . A rotary is especially suited for intersections legs, and /or where there are right- turning movements. For moderate traffic, rotaries are self-governing & need no control by police or traffic signals. The possible number of accidents and severity of accidents are quite low because of low relative speed.  It can be constructed with advantage when the number of intersecting roads is between four and seven.

4. Disadvantages of Rotary Intersections A rotary requires more land & may not be feasible in many built-up locations. Where pedestrian traffic is large, a rotary by itself is not sufficient to control traffic & has to be supplemented by traffic police . When used on high speed roads, rotaries require extremely large size. Traffic turning right has to travel a little extra distance . A rotary requires many warning & directional signs for safety . The central island & entrances & exists must be well lighted at night. These tend to make it costly. In places where there is mixed traffic and large number of cyclists and pedestrians, the design of rotary becomes too elaborate and operation and control of traffic also become complex . When there are more than seven intersecting roads, rotaries are unsuitable.

5. Guidelines for Selecting a Rotary Type of Intersection Following are the factor decide the selection of a rotary in a road crossing 1 . Traffic volume 2 . Direction of turning of traffic 3 . Availability of other junctions nearby 4. Locality

6. 1.Traffic volume A rotary can be selected, where the volumes entering from the various roads of the intersection are approximately equal. A rotary can handle efficiently about 3000 veh / hr entering from all intersecting angles. Normally , the lowest traffic volume for which rotary treatment should be considered is about 500 veh /hr.

7. 2 . Direction of turning traffic Rotaries are the best method of intersection in locations where the proportions of right turning traffic at a junction is high

8. 3. Availability of other junctions nearby A rotary junction is suitable; where there are other road junctions are nearby that there would be insufficient road length for the formation ofqueues .

9. 4. Locality Rotaries are only suitable for high traffic and these could be a good choice for moderately busy intersections in urban and sub urban areas and also sometimes rural areas . It is a good choice when there are more than four approaches to the junction.

10. Design Factors for Rotary Intersection of Roads There are many factors to be considered while designing traffic rotary as follows : Design speed 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 Channelizing islands Camber and super elevation Sight distance Lighting Traffic signs Pedestrian ways

11. Design speed It governs various elements such as radii and weaving lengths . The IRC has recommended that the design speed of 40 KMPH for rotaries in rural areas and about 30 KMPH (generally 32 KMPH) for rotaries in urban areas .

12. Shape of central island Rotary Island may be classified based on the shape of island structure and are as fallows 1 . Circular island 2 . Square island (with round edges) 3 . Elliptical island (with round edges) 4 . Rectangular rotary (with round edges) 5 . Complex rotary (with approaches)

13. Circular island

14. Square island

15. Elliptical island (with round edges )

16. Rectangular rotary (with round edges)

17. Complex rotary (with approaches)

18. Radius of Rotary Roadway

19. Radius of the central island In practice the radius of the central island may be kept slightly larger than that of the curve at entry. It is governed by rotary design speed. The value of 1.33 times the radius of entry curve is probably adequate for this purpose. Radius of central island = 1.33 times × radius of curve at entrance

20. Weaving angle and weaving lengths For smooth flow of traffic, the weaving angle should be small but not less than 15 degree . The weaving length determines the ease with which the traffic can merge and diverge. The weaving length is decided on the basis of factors such as width of weaving section, the average width of entry, total traffic and proportion of weaving traffic. Twice the values given below can serve as the upper limit .

21. Entry and Exit angles The entry angle should be 60° if possible. The exit angle should be 30° if possible . The exit angle should be small, even tangential

22. Radius at exit

23. Width of Carriageway At Entry And Exit Width of Carriageway at Entry and Exit of a rotary is governed by the movement of traffic entering and leaving the rotary. Minimum width of carriage way of 5m, with provision for extra widening due to curvature may be provided for entrance and exit .

24. Width of Rotary Carriageway Width of Rotary Carriageway consists of 2 groups 1 . Width of weaving section 2 . Width of non-weaving section The width of non-weaving section of the rotary should be equal to the widest single entry into the rotary and generally it should be less than the width of the weaving section

25. Grades: Gradient should not exceed 2 %. (For extraordinary condition, gradient is provided). Sight distance: Sight distance for the speed of about 30 KMPH to 40KMPH should be vary from 30- 45 m