Traffic geometrics:- basic geometric elements, cross roads, rotary intersections grade separated intersection, terminal facilities

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PREPARED BY : ASST. PROF. VATSAL D. PATEL
MAHATMA GANDHI INSTITUTE OF
TECHNICAL EDUCATION &
RESEARCH CENTRE, NAVSARI.

2.  Basic geometric elements of traffic geometrics are :
 1. Intersection at grade
 2. Grade separated intersections (interchanges)
 3. Traffic islands
 4. Terminal facilities
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3.  Intersection is the location on a roadway where two or more
approaches from different directions are meeting.
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4.  Highway intersection design has got a considerable influence
on safety and efficiency of traffic movements.
 Road intersections can be divided into two groups :
 1. Intersection at grade
 2. Grade separated intersection
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5.  In this type of intersection all the roads meet at more or less
the same level involved in the intersection at grade.
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Intersection at
grade
Unchannelized
Intersection
Channelized
Intersection
Rotary
Intersection

6.  1. According to shape :
 Three legged intersection : T, Y and skewed intersection
 Four legged intersection : Right angled, skewed, offset
 Rotary intersection or round abouts
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7.  2. According to Geometric design features:
 Unchannelized Intersection
 Channelized Intersection
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8.  3. According to traffic control:
 Uncontrolled intersection : No regulatory devices (signals,
signs, police) are used. Driver has to wait for a gap to merge
or cross the other traffic stream.
 Controlled intersections : Traffic is controlled by signals, signs
or police. They provide orderly movement and better level of
safety.
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9.  3. According to traffic control:
 Signalized intersection : Light signal control is used on these
intersections for movement of vehicles.
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10.  4. According to levels of crossing:
 At grade intersection
 Grade separated intersections (interchanges)
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11.  Forms of intersection at grade
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12.  The intersection area is paved and there is absolutely no
restriction to vehicle to use any part of intersection area. They
are the lowest class of intersection easiest in design, but most
complex in traffic operations resulting in maximum conflict
area and more number of accidents.
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13.  Channelized intersection is achieved by introducing islands
into the intersectional area, thus reducing the total conflict area
available. These islands help to channelize turning traffic, to
control their speed and angle of approach and to decrease the
conflict area at the intersection.
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14.  By channelization vehicles can be confined to definite paths.
 Conflict areas within the intersection can considerably be
decreased.
 Angle of merging streams can be forced to be flat angles so as
to cause minimum disruption.
 Angle between intersecting streams of traffic may be kept as a
desired in a favorable way.
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15.  Speed of vehicles approaching the intersection can be
controlled.
 Refuse islands can be provided for pedestrians.
 Points of conflicts can be separated.
 Signs and other traffic control devices can be installed in the
traffic islands.
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16.  Traffic islands are raised areas constructed within the roadway
to establish physical channels through which the vehicular
traffic may be guided.
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17.  To segregate the pedestrians and vehicles.
 To separate traffic into specified paths.
 To reduce the conflict area to minimize hazards.
 To increase safety.
 To divert traffic from obstacles and expedite the traffic flow.
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18.  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.
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19.  It consumes useful traffic area.
 It increases travel distance.
 Width of traffic lane decreases.
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20.  Traffic islands are classified as :
 1. Divisional islands
 2. Channellizing islands
 3. Pedestrian loading islands
 4. Rotary islands
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21.  These are intended to separate opposing traffic flow on a
highway with four or more lanes. By thus dividing the
highway into two one-way roadways, the head on collisions
are eliminated.
 The width of the divisional islands should be large if the head
light glare is to be reduced during night driving. The kerb
should be high enough to prevent vehicles from entering into
the islands.
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22.  These are used to guide the traffic into proper channel through
the intersectional area.
 Channelizing islands are very useful as traffic control devices
for intersection at grade.
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23.  The area of possible conflicts between traffic stream is
reduced.
 They are useful when the direction of flow is to be changed.
 They serve as convenient locations for other traffic control
devices.
 They serve as refuge islands for pedestrians.
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24.  They are provided at regular bus stops and similar places for
the protection of passengers.
 A pedestrian islands at or near a cross walk to aid and protect
pedestrian crossing the carriage way may be termed as
pedestrian refuse islands.
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25.  It is a large central island of a rotary intersection.
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26.  The various types of conflicts at an intersection are :
 Crossing conflicts
 Merging conflicts
 Diverging conflicts
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27.  A rotary intersection is an enlarged road intersection where all
converging vehicles are forced to move round a large central
islands in one direction (clockwise direction) before they can
weave out of traffic flow into their respective directions
radiating from the central island.
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28.  The main objects of providing a rotary are to eliminate the
necessity of stopping even for crossing streams of vehicles and
to reduce the area of conflict.
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29.  It ensures one way movement of traffic.
 Frequent stopping and starting are avoided.
 All traffic proceeds simultaneously and continuously at
uniform speed.
 It reduces crossing conflicts from 16 to 4.
 There is no need of traffic police or signals at traffic rotary.
 Maintenance cost of traffic rotary is negligible.
 It provides easy movement for right turn traffic.
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30.  It requires large area for effective operations.
 It does not provide any provision for pedestrian.
 It is difficult to separate slow moving traffic.
 It is not suitable where angle of intersection between two
roads is too acute.
 It requires many warning and directional signs for safety.
 Traffic turning right has to travel a little extra distance.
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31.  Rotaries are most adopted where volumes entering from
different intersection legs are nearly equal.
 The maximum practical capacity of high type rotaries may be
equal to total volume of 3000 vehicles per hour entering from
all the intersection legs.
 When there are more than 4 approaches to the junction, a
rotary is preferred.
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32.  A rotary design is most appropriate when the proportion of
turning traffic is very high.
 Rotaries are not generally warranted for intersections carrying
very light traffic volume. Minimum traffic volume of 500
vehicles per hour is required.
 When the approach widths are restricted and it is impossible to
provide separate lanes for through and turning traffic, a rotary
may be desirable.
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33.  Design speed
 Shape of central island
 Radius at 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
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34.  Design speed :
 The vehicles approaching at intersection at grade have to
considerably slow down their speed when compared to the
design speed of the highway.
 Current Indian practice is to design rotaries for the following
speeds :
 In rural areas – 40 km.ph
 In urban areas – 30 km.ph
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35.  Shape of central island :
 The shape of the central island depends on the number and
layout of the intersecting roads. The outline of the central
island consists of a number of curves of large radii, without
corners.
 The various shapes considered to suit different conditions are
circular, elliptical, turbine and tangent shapes.
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36.  Shape of central island :
 When two equally important cross roads at roughly right
angles, a circular shape is suitable. The island may be often
elongated to accommodate in the layout four or more
intersecting roads.
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37.  Radius at entry :
 The radius at entry is determined by the design speed, super
elevation and coefficient of friction.
 Adequate super elevation cannot be provided on the rotary
roads and hence it is safer to neglect the super elevation and to
take friction only into consideration.
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38.  Radius at exit :
 The exit radius should be higher than the radius of the rotary
island so that it favors a higher speed by drivers.
 The general practice is to keep the radius of exit curves 1.5 to
2 times the radius of the entry curves.
 In case of large pedestrian traffic across the exit road, the
radius of exit curve may be taken equal to the radius of entry
curve to keep the exit speeds reasonably slow.
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39.  Radius of central island :
 The radius of central island may be kept slightly larger than
the radius of entry curve.
 The recommended mini radii of central island are 1.33 times
the radius of entry curves.
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40.  Wearing length :
 The angle between the path of a vehicle entering the rotary
and that of another vehicle leaving the rotary at adjacent road,
thus crossing the path of each other is termed as wearing
angle.
 For smooth traffic flow the wearing angle should be small but
not less than 12°.
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41.  Width of carriage way at entry and exit :
 The width of carriage way at entry and exit of a rotary is
governed by the amount of traffic entering the rotary or
leaving the rotary.
 A minimum width of carriageway at entry and exit is 5.0 m.
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42.  Width of rotary carriageway :
 The width of non-weaving section of the rotary should be
equal to the widest single entry to the rotary and should
generally be less than the width of weaving section.
 The width of weaving section (w) should be one traffic lane
(3.5 m) wider than the mean entry width.
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43.  Entry and exit angles :
 Entry angles should be larger than exit angles. It is desirable
that the entry angles should be about 60°, and exit angle 30°.
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44.  Camber and super elevation :
 The rotary curvature being opposite to that of entry and exit,
the super elevation in the two portions will be opposite to each
other.
 A vehicle passing along a rotary traverses a reserve curve
while changing from one way path of roadway to the exit of
the radial road.
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45.  Camber and super elevation :
 To cause the least discomfort, the algebraic difference in the
cross-slopes should be limited to about 0.07.
 The inward slope of the camber serves as super elevation for
the traffic going round the central island and the outer slope of
camber helps the vehicles turning left towards the exit curve to
the radiating road.
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46.  Capacity :
 The capacity of a rotary is directly determined by the capacity
of each weaving section. The capacity of a weaving section is
determined by the geometric layout, including entrances and
exits, and percentage of weaving traffic.
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47.  In a grade separated intersection (interchange) the intersecting
roadways are placed at different elevations to allow
uninterrupted traffic movement.
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48.  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 an 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.
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49.  It avoids necessity of stopping.
 Avoids accidents at intersections.
 There is increased safety or turning traffic.
 There is overall increase in comfort and convenience to the
motorists and saving in travel time.
 The capacity of the grade separated intersection can practically
approach that of the two cross roads.
 It is very useful for expressways.
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50.  It is very costly.
 Where there is limited right of way like built up or urban area
or where topography is not favorable, construction of grade
separation is costly, difficult and undesirable.
 In plain terrain, grade separation may introduce undesirable
crests and sags in the vertical alignment.
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51.  On highway facilities such as expressways, freeways and
motorways.
 Certain at-grade intersections which have reached the
maximum capacity and where it is not possible to improve the
capacity further.
 At certain locations of at-grade intersections the accident rate
is very high.
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52.  At junctions where the traffic volume is heavy and the delays
and loss caused justify economically the provision of grade
separation.
 At certain topographical situations where at grade intersection
involve considerable earthwork or acquisition of land.
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53.  Along the highway the terminal facilities required are :
 1. Bus stops
 2. Bus bays
 3. Drive ways
 4. Footways
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54.  Bus stops :
 The guidelines followed for the location of bus stops are :
 It is desirable that the bus stops should be located 75 m away
from the intersection, so that bus traffic does not interferes
with the traffic flow.
 It is preferable to locate the bus stop on the exit side of the
intersection.
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55.  Bus stops :
 Bus stops on opposite sides of two-way undivided carriage
ways should be staggered so that buses stop tail to tail and
move off from each other. A stagger suggested is 60-90 m.
 Bus stops should be so located as to disembark the passengers
at safe places such as kerbs or islands.
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56.  Bus bays :
 It is desirable to recess the curb sufficiently for
accommodating the bus-bays. Properly designed bus bays
facilitate loading and unloading of passengers without the
vehicle blocking the stream of traffic on the carriageway.
 The length of recess should be about 12-15 m for a single bus
with an additional length of 12-15 m for every additional bus.
The taper on either side should be about 8:1 with max. of 6:1.
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57.  Drive ways :
 Driveways are roads providing access to abutting property
owners and utilities.
 An access road to a petrol filling station is an example of drive
ways.
 The entrance and exit should be wide with easy curves.
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