1. INTRODUCTION
Raised areas constructed within the roadway to establish physical channels through which the vehicular traffic may be guided.
Within an intersection area, a median or an outer separation is considered to be an island.
Traffic islands include all areas created for separating and directing vehicular traffic.

2. Classification of Islands
Based on function ,islands are classified as
Channelizing Islands
Divisional Islands
Pedestrian Loading Islands
Rotary Islands

3. Channelizing Islands
For guiding traffic into proper channel at intersection.
Designed to control and direct traffic movement, usually turning.

4. Channelizing Islands
Installed in areas to bring about an orderly flow of traffic.
Serve as location for other traffic control devices, refuge islands for pedestrians.
Channelization is particularly helpful at streets intersecting at oblique angles, at 3-leg junctions, and at multileg intersections.
Traffic channelizing islands may be provided for separation (and special control) of turning movements.
Size & shape of island depends upon layout and dimensions of intersection.

5. Divisional Islands
Designed to separate opposing flow of traffic streams.
Eliminate head-on collision.

6. Divisional Islands
Divisional islands are used to guide traffic around an obstruction within the roadway in advance of an intersection to separate opposing traffic .
Located to prevent overtaking and passing at hazardous points, such as sharp curves or narrow underpasses.
Width of islands should be large to prevent headlight glare.
It should be high to prevent vehicles from entering into islands.

7. Divisional Islands
Where divisional islands are continuous, they are called medians; the more important functions are as follows:
1. Medians provide an insulating area between opposing streams of moving traffic.
2. Medians provide protection and control of cross and turning traffic.
3. Medians provide a refuge for pedestrians.

8. pedestrian loading islands ..
Pedestrian islands are provided to serve as safety zones for the aid and protection of persons on foot.
If a divisional island is located in an urban area where pedestrians are present, portions of each island can be considered a refuge island.

9. Refuge islands
Pedestrian islands are provided to serve as safety zones for the aid and protection of persons on foot.
Refuge islands are particularly useful at intersections in urban areas where
There is a considerable amount of pedestrian traffic
Where heavy volumes of vehicular traffic make it difficult and dangerous for pedestrians to cross

10. Rotary Island Large central island of a rotary intersection
Much larger than the central island of channelized intersection
Crossing manoeuvre is converted to weaving by providing sufficient weaving length

11. Guidelines for selection of island type
Traffic characteristics at the intersection
Cost considerations, and
Maintenance needs

13. INTERSECTIONS

14. TYPES OF INTERSECTIONS
INTERSECTION-General area where two or more highways join or cross
Intersection Leg- roadway radiating from an intersection
Two types
Intersection at grade
Grade seperated intersection

15. INTERSECTION AT GRADE

16. INTERSECTION AT GRADE
An intersection where all roadways join or cross at the same level.
Allowing traffic manoeuvres like merging, crossing, and weaving
Classified as:
UNCHANNELIZED
CHANNELIZED
ROTARY INTERSECTIONS

17. Forms of Intersections ..
T intersection
Cross
Staggered
Skewed

18. Forms of Intersections
Skewed cross
Skewed staggered
Wye
Multiple

19. Channelization
Channelized intersection is achieved by introducing islands into intersection area thus reducing total conflict area .
Direction of traffic flow at intersections to definite paths by means of traffic markings, islands or other means
Islands helps to channelize turning traffic, control speed and angle of approach and to decrease conflict area at intersection.

20. Purpose Separation of conflicts Control of angle of conflict
Control of speed
Protection of traffic for vehicles leaving or crossing the main traffic stream
Protection of pedestrians
Elimination of excessive intersectional areas
Blockage of prohibited movements
Location of traffic control devices

21. Features
Many different shapes and sizes guided by geometry of intersection
Sufficiently large to command attention
5 m2 or preferably 7 m2 – minimum
Elongated or divisional islands – Introduced on undivided highways to alert drivers and regulate traffic through the intersections should be atleast 1.2 m wide and 3.5 to 6 m long

22. ROTARY INTERSECTIONS OR ROUND ABOUTS
Specialised form of at-grade intersection laid out for movement of traffic in one direction round a central island
Major conflicts at an intersection - collision between through and right-turn movements
Vehicles from converging area are forced to move around central island in clockwise direction in an orderly manner and weave out of the rotary into their desired directions.

23. Rotary Intersection

24. Advantages .. Orderly traffic flow
Traffic proceeds simultaneously and continuously at fairly uniform, though low speed
Frequent stopping and starting are avoided
Weaving movements replace the usual angular crossing of typical at-grade intersection
Direct conflict is eliminated, all traffic streams merging off leaving at small angles
Reduce accidents and their severity .

25. Advantages ..
Traffic flow is regulated to only one direction of movement, thus eliminating severe conflicts between crossing movements.
All the vehicles entering the rotary are gently forced to reduce the speed and continue to move at slower speed.
None of the vehicles need to be stopped, unlike in a signalized intersection.
Rotaries are self governing and do not need practically any control by police or traffic signals.
Ideally suited for moderate traffic, especially with irregular geometry, or intersections with more than three or four approaches.

26. Disadvantages
On high speed roads, rotaries require extremely large size
When provided at close intervals, they make travel troublesome
Traffic turning right has to travel a little extra distance
As the flow increases and reaches the capacity, weaving generally gives way to a stop and go motion as vehicles force their way into the rotary, being followed by vehicle waiting in the queue behind them.
High traffic and pedestrians makes rotary operation complex.
When pedestrian traffic is large, a rotary by itself is not sufficient to control traffic and has to be supplemented by traffic police

27. Traffic Operations in a Rotary

28. Rotary design elements

29. DESIGN SPEED
All the vehicles are required to reduce their speed at a rotary.
Therefore, the design speed of a rotary will be much lower than the roads leading to it.
Although it is possible to design roundabout without much speed reduction, the geometry may lead to very large size incurring huge cost of construction.
Design speed n rural area is 40 kmph and in urban area is 30 kmph.

30. SHAPE OF CENTRAL ISLAND
Shape depends on number and layout of intersecting roads.
Various shapes are
Circular-when two equally important roads cross at right angles
Elongated- accommodate four or more intersecting roads and to allow greater traffic flow.
Turbine- forces reduction of speeds of vehicles entering and speeding up of vehicles going out.

31. ENTRY RADIUS
Radius at the entry depends on various factors like design speed, super-elevation, and coefficient of friction.
Entry to the rotary is not straight, but a small curvature is introduced.
This will force the driver to reduce the speed (design speed of rotary)
Entry radius of about 15-20m for urban area and 20-35m for rural design.

32. EXIT RADIUS
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 speed.
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.

33. Radius of Central Island
Governed by the design speed, and the radius of the entry curve.
Radius of the central island, is slightly higher than that of entry radius
Radius of the central island is about 1.33 times that of the entry curve

34. WIDTH OF THE ROTARY
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.
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.
Minimum width at entry & exit should be 5m.

35. WEAVING LENGTH
Determines how smoothly the traffic can merge and diverge.
Decided based on many factors such as
Weaving width,
Average width of entry
Traffic.

36. ENTRANCE AND EXIT CURVES
Vehicles leaving rotary should accelerate the speed. Hence exit radius should be of large radius.
Normal pavement width at entrance and exit should be equivalent to two lanes.
Extra -widening is provided at both entry and exit curve

37. CHANNELIZING ISLANDS
Channelizing Islands are provided at both entrance & exit
Size and shape of islands depends upon radius of rotary and radius of entry and exit curves

38. SIGHT DISTANCE
Sight distance in the rotary should be as large as possible.
Minimum sight distance should be 45 m for design speed of 40kmph and 30m for 30 kmph.

39. GRADE SEPERATED INTERSECTION

40. GRADE SEPERATED INTERSECTION
Highest form of intersection treatment
Causes least delay and hazard to the crossing traffic
Grade seperation may be either by an overbridge or
under pass

41. OVERPASS
When major highway is taken above by raising its profile above general ground by embankment and an over bridge across another highway.
ADVANTAGES
Reduce drainage problems
Aesthetic preference to main traffic
Less feeling of restriction compared to underpass
Future construction or expansion of separate bridge structure for divided highway is possible.

42. OVERPASS
DISADVANTAGES
In rolling terrain if major road is taken above the vertical profile also changes.
Increased grade resistance may cause speed reduction on heavy vehicles
Restrictions to sight distance may occur.

43. UNDERPASS
Highway is taken by depressing it below ground level .
Choice of an overpass or underpass depends on topography, vertical alignment, drainage, economy, aesthetics etc.

44. UNDERPASS
ADVANTAGES
Provide warning to traffic in advance due to presence of underpass which can be seen from distance.
When major highway is taken below ,advantage to turning traffic because traffic from cross road can accelerate while descending the ramp to major highway.
Traffic from major highway can decelerate while ascending the ramp to cross roads.
Main advantage , when main highway is along existing grade without alteration of vertical alignment and cross road is depressed.

45. UNDERPASS
DISADVANTAGES
Drainage problems during rainy season when underpass is depressed upto 5-7m below ground level.
Necessary to pump water continuously.
Overhead structure may restrict sight distance
Feeling of restriction to traffic .