Prof. Mehul M Chavda
Design Geometric
Design intersection & Traffic
Island

Introduction
1. Intersection at grades
2. Grade separated intersections
3. Traffic island
4. Terminal facilities

Intersections
• Intersection is the location on a roadway
where two or more approaches from different
directions are meeting.
1. Intersection at Grade
2. Grade separated Intersection

INTERSECTION AT GRADE
• Unchannelized Intersection
• Channelized Intersection
• Rotary Intersection

Classification of Intersection at Grade
A) According to Shape:-
Three legged intersection (T, Y and skewed)
Four legged intersection (Right angled, skewed, offset)
Rotary intersection
B) According to Geometric design features:-
Unchannalized intersection
Channelized intersection
C) According to traffic control:-
Uncontrolled intersection
Controlled Intersection
Signalized intersection
D) According to levels of crossing:-
At grade intersections
Grade separated intersections

Requirements of intersection at
Grade
• At the intersection the area of conflict should be
as small as possible.
• For the vehicles, adequate visibility should be
available.
• The relative speed and angle of approach of
vehicles should be small.
• Sudden change of path should be avoided.
• Proper signs should be provided.
• Proper lighting should be provided for better
visibility at night.

Unchannelized Intersections
• The intersection area is paved and there is
absolutely no restriction to vehicle to use any
part of intersection area.
• Lowest class of intersection
• Easy to design but most complex in traffic
operations resulting in maximum conflict area
and more no of accidents.

Channelized Intersections
• Channelized Intersections is achieved by
introducing islands into the intersectional
area, thus reducing total conflict area
available.
• These are help to channelize turning traffic, to
control their speed and to decrease conflict
area.

Advantages of Channelized
Intersections
• By channelization, vehicles confined to definite
path.
• Conflict areas can be decreased.
• Angle of merging streams can be forced to be flat
angles so as to cause minimum disruption.
• Speed of vehicle can be controlled.
• Refuse islands can be provided for pedestrians.
• Points of conflicts can be separated.
• Signs & traffic devices can be installed.

TRAFFIC ISLANDS
• Traffic islands are raised areas constructed
within the roadway to establish physical
channels through which the vehicular traffic
may be guided.

FUNCTIONS OF TRAFFIC ISLANDS
• Functions of Traffic Islands:-
• To segregate the pedestrians and vehicles.
• To separate traffic into specified paths.
• To reduce conflict areas.
• To increase safety
• To divert traffic from obstacles.

ADVANTAGES OF TRAFFIC
ISLANDS
• It provides smooth and controlled traffic flow.
• It reduces the accident and increase safety
• It gives uniform speed of traffic.
• No need of police control.

DISADVANTAGES OF TRAFFIC
ISLANDS
• It consumes useful traffic area.
• It increases travel distance.
• Width of traffic lane decreases.

TYPES OF TRAFFIC ISLANDS
1. Divisional islands
2. Channelizing islands
3. Pedestrian loading islands
4. Rotary islands

Conflicts at an Intersection
 Crossing conflicts
 Merging conflicts
 Diverging conflicts

VISIBILITY AT INTERSECTIONS
 At intersection where the intersecting roads are of
more or less equal importance & there is no
established priority, visibility should be provided .
 SSD is given by,
 SSD = lag distance + breaking distance

SSD values for different speeds
DESIGN SPEED (km.ph) SAFE SSD (m)
20 20
25 25
30 30
40 45
50 60
60 80
65 90
80 120
100 180

 It is customary to control traffic on the minor road by
STOP or GIVEWAYsign or road markings.
 Such intersection are known as Priority Intersection.
 Priority to the right is a right-of-way system, in which the
driver of a vehicle is required to give way to vehicles
approaching from the right at intersections.

WIDTH OF CARRIAGEWAY
 Width needs to be increased in curves, keeping in mind
mechanical widening and psychological widening.
 Extra Widening = Mechanical Widening +
Psychological Widening
 We = nl ²/2R + V/9.5(R)½

MEDIAN OPENINGS
 At Intersections, where nearly all the traffic travels on the
divided highway and volume is below capacity, the
occasional vehicles may use the median opening either to
come to the through road from the cross road or reverse.
 A bullet nosed median opening designed for the minimum
turning path will meet the requirements.
 A bullet nosed median end is preferable.
 The length of opening depends on the width of median.

Median opening with bullet nosed end

T-INTERSECTION
 It is normally provided for very light traffic and for minor
and local roads.
 For locations where speeds are high and turning
movements are large, it is hazardous.

T-INTERSECTION

Y-INTERSECTION
 Mostly design features are similar to that of T-
Intersections, except where the intersection angle is very
acute.

Y-INTERSECTION

CROSS ROADS
 In case of cross roads, two roads crosses approximately at
right angles.
 Uncontrolled cross roads are very dangerous due to high
accident rate.

CROSS ROADS

STAGGERED INTERSECTION
 The safety at the cross roads can be significantly enhanced
by converting it into a staggered intersection.
 A minimum distance of 35 m is necessary between the
staggered roads.
 A right/left staggered is preferred to left/right stagger
where there are no separate right turning lanes.

STAGGERED INTERSECTION

STAGGERED INTERSECTION

ROTARY INTERSECTION
 A ROTARY INTERSECTION is an enlarged road
intersection where all converging vehicles are forced to
move round a large central island in one direction before
they can weave out of traffic flow into their respective
directions radiating from the central island.
 The main object of providing a rotary are to eliminate the
necessity of stopping even for crossing streams of vehicles
and to reduce area of conflict.

Advantages of rotary intersection
• It ensure one way traffic movement
• Frequent starting and stopping are avoided
• Vehicles move at uniform speed
• It reduces crossing conflicts from 16 to4
• No need of traffic police or signal
• Maintenance is negligible
• Easy movement of right turn traffic

Disadvantages of rotary intersection
• It require large area
• It does not provide any provision for
pedestrian
• Difficult to separate slow traffic movement
• It require may warning and direction sign for
safety

ROTARY DESIGN ELEMENTS
Design Speed
Shape of central island
Radius of entry
Radius at exit
Radius of central island
Weaving length
Width of carriage way at entry and exit
Width of rotary carriageway
Entry and exit angles
Camber and super elevation
Capacity

GRADE SEPARATED INTERSECTIONS
 In a grade separated intersection, the intersecting roadways
are placed at different elevations to allow uninterrupted
traffic movement.
 The grade separated intersections are classified as
OVERPASS and UNDERPASS.
 When the major highway is taken over the road of lesser
importance, it is called as OVERPASS.
 If the major highway is taken by depressing it below the
ground level to cross another road of less importance by
means of under bridge, it is known as UNDERPASS.

TRUMPET INTERSECTIONS

DIAMOND INTERCHANGE

PARTIAL CLOVER LEAF

CLOVER LEAF INTERCHANGE

ADVANTAGES OF GRADE
SEPARATION
It avoids necessity of stopping.
Avoids accidents at intersection.
There is increased safety or turning traffic.
There is overall increase in comfort and convenience to the
motorists and saving in travel time.
It is very useful for expressways.

DISADVANTAGES OF GRADE
SEPARATION
It is very costly.
Where there is limited right of way like built up or urban area
or where topography is not favourable, construction of grade
separation is costly, difficult and undesirable.

TYPES OF GRADE SEPARATED INTERSECTION
 Grade separated intersections without interchange.
 Grade separated intersections with interchange.
 Depending upon no of legs the intersection serves, they are classified
as-
 A) Three-legged Interchange
 T-Intersection
 Y-Intersection
 Partial rotary interchange
B) Four legged interchange
 Diamond Interchange
 Clover leaf interchange
 Rotary Interchange
 Directional Interchange
C) Multi-legged interchange
 Rotary interchange

ROTARY INTERCHANGE

DIRECTIONAL INTERCHANGE

TERMINAL FACILITIES
 Along the highway the terminal facilities required are:-
Bus stops
Bus bays
Drive ways
Footways

• Thank You