Road Intersection Design Guide

Intersection Designs
By
Er. Harpreet Singh
1By Er. Harpreet Singh

Intersection of road
 Intersection is an area shared by two or more
roads.
 This area is designated for the vehicles to turn
to different directions to reach their desired
destinations.
 This is because vehicles moving in different
direction want to occupy same space at the
same time.

Types
 There are two main types of intersection of
roads.
 Grade – separated intersections or
interchanges.
 At – grade intersections.

Grade separated intersection or interchanges
 It is a bridge that eliminates crossing conflicts at intersections
by vertical separation of roadways in space.
 Route transfer at grade separations is accommodated by
interchange facilities consisting of ramps.
 The interchange configurations are designed in such a way to
accommodate economically the traffic requirements of flow,
operation on the crossing facilities, physical requirements of
the topography, adjoining land use, type of controls, right-of-
way and direction of movements.

Grade separated intersection or
interchanges
Objective:
 The ultimate objective of grade separated
intersections is to eliminate all grade
crossing conflicts and to accommodate
other intersecting maneuvers by merging,
diverging and weaving at low relative seed.

Classification of Grade Separated
Intersection
 One of the distinctions made in type of
interchange is between the directional and the
non directional.
 Directional interchanges are those having
ramps that tend to follow the natural direction
of movement.
 Non directional interchanges require a change
in the natural path of traffic flow.

Major Interchanges
 Underpass
 Overpass
 Trumpet Interchange
 Diamond Interchange
 Cloverleaf Interchange
 Partial cloverleaf Interchange
 Directional Interchange
 Bridged Rotary

Underpass
 An underpass or a tunnel is an underground
passageway, completely enclosed expect
for openings for ingress and egress,
commonly at each end.
 A tunnel may be for foot or vehicular road
traffic, for rail traffic.

Overpass
 An overpass also known as a flyover, is a
bridge, road, railway or similar structure
that crosses over another road or railway.
 A pedestrian overpass allows pedestrians
safe crossing over busy without impacting
traffic.

Trumpet interchanges
 Trumpet interchanges have been used where one
highway terminates at another highway.
 These involve at least one loop ramp connecting
traffic either entering or leaving the terminating
expressway with the far lanes of the continuous
highway.
 The principal advantages are low construction cost
and are useful for highways as well as toll roads.

Diamond interchange
 A diamond interchange is a common type of road
junction, used where a freeway crosses a minor
road.
 The diamond interchange uses less space than most
types of freeway interchange, and avoids the
interweaving traffic flows that occur in
interchanges such as the cloverleaf.
 Diamond interchanges are most effective in areas
where traffic is light.

Cloverleaf interchange
 A cloverleaf interchange is a two-level
interchange in which left turns are handled by
ramp roads.
 To go left(in right-hand traffic), vehicles first
continue as one road passes over or under the
other, then exit right onto a one-way three-
fourths loop ramp (270) and merge onto the
intersecting road.

Partial cloverleaf interchange
 Partial clover leaf is a modification that
combines some elements of a diamond
interchange with one or more lops of a
cloverleaf to eliminate only the more
critical turning conflicts.
 It provides more acceleration and
deceleration space on the freeway.

Directional interchange
 A Directional interchange provides direct for left
turns.
 These interchanges contain ramps for one or more
direct or semi direct left turning movements.
 Interchanges of two freeways or interchanges with
one or more very heavy turning movements usually
warrant direct ramps, which have higher speeds of
operation and higher capacities, compared to loop
ramps.

At grade intersections
 At-grade intersections in which all the
exchanges between the roads take place on
the sane plane.
 These are of two types
 Standard at-grade intersections
 Round about at-grade intersections.

Simple intersections
 Simple intersections maintain the street’s
typical cross-section and number of lanes
throughout the intersection, on both the major
and minor streets.
 Simple intersections are best-suited to
locations where auxiliary (turning) lanes are
not needed to achieve the desired level-of-
service.

Channelized intersections
 Vehicles approaching an intersection are directed to definite
paths by islands, marking etc. and this method of control is
called channelization.
 Channelized intersection provides more safety and efficiency.
 It reduces the number of possible conflicts by reducing the area
of conflicts available in the carriageway.
 If no channelizing is provided the driver will have less
tendency to reduce the speed while entering the intersection
from the carriageway.

Design parameters of
intersections

Pedestrians
 Crossings and pedestrian curb cut Ramp
Locations
 Walking Speed
 Pedestrian Flow Capacity
 Traffic Control, Yielding and Delay

Bicyclists
 Cross-section- Bicyclists position themselves
for their intended destination regardless of the
presence of bike lanes or shoulders.
 Operating Speed- At un signalized
intersections, an average bicycle speed of 15
miles per hour can be assumed on the major
project.

Bicyclists
 Bicycle Capacity - The number of bicycles
per hour that can be accommodated by the
facility under normal conditions.
 Traffic Control - Bicyclists are required by
law to obey control devices at intersections.
Therefore, traffic control devices need to
account for bicycle activity.

Motor vehicles
Design Vehicle
*The largest type of motor vehicle that is normally expected to be
accommodated through the intersection.
*At intersections, the most important attribute of design vehicles is their
turning radius, which in turn influences the pavement corner radius
and therefore the size of the intersection.
*Lane width, another feature related to the design vehicle, has some
impact on intersection design.
* The design vehicle appropriate for most types of transit service is the
“city-Bus” as defined by AASHTO.

Levels of intersection control
Passive control
> No control
> Traffic signs
> Traffic signs plus marking.

Levels of intersection control
Semi control
 Channelization
 Traffic rotaries
Active control
 Traffic signals
 Grade separated intersections

Unchannelized Intersections
 Intersection area is paved and there is absolutely no
restriction to vehicles to use any part of intersection area.
 Hence the un channelized (all-paved) intersections are the
lowest class of intersection, easiest in the design but most
complex in traffic operations
 Resulting in maximum conflict area and more number of
accidents, unless controlled by traffic signals or police.

Types of unchannelized intersection:
 Plain intersection:
No provision for additional
pavement width for turning
movements
 Flared intersection:
Provision for additional
pavement width for turning
movements

Traffic island A small raised area in the middle of a road which
provides a safe place for pedestrians to stand and
marks a division between two opposing streams of
traffic.
(or)
 A traffic island is a solid or painted object in a road
that channelizes traffic. It can also be a narrow strip of
island between roads that intersect at an acute angle.
 If the island uses road markings only, without raised
kerbs or other physical obstructions, it is called
a painted island. Traffic islands can be used to reduce
the speed of vehicles driving through.

Types of Traffic islands Divisional Islands: Divisional islands are dividing the highway
in two one way road way so that head on collision are eliminated
and accidents are reduced.
 Channelizing Islands: Channelizing islands guide the traffic
into proper channel through the intersection area.
 Pedestrian loading Islands: They are provided at regular bus
stops and similar laces for the protection of passengers.
 Rotary Islands: Rotary islands is the large central island of a
rotary intersection.

Rotary Islands
 Rotary intersections or round
abouts are special form of at-
grade intersections laid out
for the movement of traffic in
one direction around a central
traffic island.
 The vehicles entering the
rotary are gently forced to
move in a clockwise direction
in orderly fashion.

Width of the Rotary
 IRC suggest that a two-lane of 7m width should be kept as 7m for
urban roads and 6.5 for rural roads.
Further for a three-lane road ofb10.5m is to be reduced to 7m and 7.5m
respectively for urban and rural roads.
 The width of weaving section should be higher than the width at
entry and exit. The weaving with is given as,
Where e1 = width of carriageway at the entry
e2 = width of carriageway at the exit
W weaving= {(e1+e2)/2} +3.5 m

Capacity The capacity of rotary is determined by the capacity of each
weaving section. Transportation road research lab (TRL) proposed
the following empirical formula to find the capacity of the weaving
section.
 where is the average entry and exit width, is the weaving width, is
the length of weaving, and is the proportion of weaving traffic to
the non-weaving traffic.
 four types of movements at a weaving section, and are the non-
weaving traffic and are the weaving traffic.

New Concept to Design Rotary at
Road Intersections “Determine the radius of a circle in which
lengths of two chords ‘a’ and ‘b’ are known
and the chords meet at any point on the
periphery of the circle” ?..
 The equation derived as solution of the
problem is given below. It is a quadratic
equation in terms of R2.

Design Consideration
i. Length of Intersecting straights with minimum site distances
recommended by I.R.C. are: 50 km/h.,
ii. Minimum Sight distance 15 m along minor roads S.K. Mahajan et
al. / Procedia - Social and Behavioral Sciences 96 ( 2013 ) 2791 –
2799 2795
iii. Weaving Distance = 110 m, 145 m, 180 m and 220 m for speeds
50 km/h., 65 km/h., 80 km/h, 100 km/h. respectively. Minimum
Weaving length: 30 to 50 m
iv. All traffic islands are constructed 200 mm higher than road level.
They are provided with lawn, statue or fountains, painted with
colours in contrast with the road pavement.
{1-(a/b)2} R4-{b2-5a2/4} R2 –(a2b2/2) = 0

Road Intersection Design Guide

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