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.

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

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

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.

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

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.

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.

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.

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

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

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

INTERSECTIONS

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

INTERSECTION AT GRADE

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

Forms of Intersections .. T intersection Cross Staggered Skewed

Forms of Intersections Skewed cross Skewed staggered Wye Multiple

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.

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

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

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.

Rotary Intersection

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 .

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.

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

Traffic Operations in a Rotary

Rotary design elements

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.

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.

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.

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.

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

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.

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.

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

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

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.

GRADE SEPERATED INTERSECTION

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

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.

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.

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.

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.

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 .