Traffic problems are a bugbear in most metropolises
around the world. Statistics are routinely circulated about the city worst
hit by traffic congestion . However, even periodic traffic surveys and
studies, as well as the imposition of regulations fail to yield appropriate
solutions. In this grim scenario, the transportation engineer came upwith
grade separators concept, becoming very common now.
Grade separation is the process of aligning
a junction of two or more transport axes at different heights
(grades) so that they will not disrupt the traffic flow on other
transit routes when they cross each other.
The composition of such transport axes does not have to be
uniform; it can consist of a mixture of roads, footpaths, railways,
canals, or airport runways. Bridges, tunnels, or a combination of
both can be built at a junction to achieve the needed grade
The grade seperators are mainly divided in to two categories i.e.
OVERPASSES term used to describe the roads
crossing over any other road. It can be a bridge, flyover or any
other similar structure.
UNDERPASSES used to describe a passage
underneath something, especially a section of road that passes
under another road or a railroad. An intersection formed in this
A grade separator can either be a
flyover or an underpass, that is any
structure which separates the
uniform grade of a running highway
into two different grades.
A bridge is a structure built to span a
gorge, valley, road, railroad track,
river, body of water, or any other
physical obstacle, for the purpose of
providing passage over the obstacle.
Grade separators can be bridges, but
not all bridges are grade separators.
• An example of the
potential complexity of
grade separation, seen
in the Circle
Interchange in Chicago
These junctions connect two roads :
Stack interchange :- two-level, three-
level, or four-level stack, depending
on how many levels cross at the
Cloverleaf interchange :- A cloverleaf
interchange is a two-level interchange
in which left turns (in countries that
drive on the right) are handled by
Compact grade-separation:- whereby
the two roads are linked by a compact
"connector road", with major-minor
priority junctions at each of its ends;
usually a variant of the cloverleaf type
interchange, but only involving two
quadrants rather than four.
• 4 level stack interchange
between the M25 (in the
foreground) and M23 in
These junctions connect two roads, but only one is fully grade-
separated, i.e. traffic on one road does not have to stop at yield lines
or signals on one road, but may have to do so when switching to
• Diamond interchange
• Parclo interchange
• Single-point urban interchange
• Roundabout interchange
These junctions terminate one road into
• Trumpet interchange
• Directional-T interchange
• A stack interchange is a four-way interchange whereby left turns
are handled by semi-directional flyover/under ramps. To go left
(right in countries with left-hand drive), vehicles first turn
slightly right (on a right-turn off-ramp) to exit, then complete
the turn via a ramp which crosses both highways, eventually
merging with the right-turn on-ramp traffic from the opposite
quadrant of the interchange.
• A stack interchange, then, has two pairs of left-turning ramps, of
which can be stacked in various configurations above or below
the two interchanging highways.
• Stacks do not suffer from the problem of weaving, but require
massive construction work for their flyovers. A standard stack
interchange includes roads on four levels. This is not only
expensive, but also creates an eyesore among local residents.
Six-level urban stack
interchange in Puxi, Shanghai,
China (Nanbei Elevated Road at
Yanan Middle Road)
The High Five
is a five level
• A cloverleaf interchange is a
two-level interchange in which
left turns (in countries that drive
on the right) are handled by
loop roads (U.S.: ramps, UK: slip
• To go left (in right-hand
traffic)vehicles first pass either
over or under the other road,
then turn right onto a one-way
three-fourths loop ramp (270°)
and merge onto the intersecting
A typical cloverleaf
• A point of conflict is the merging of exiting and entering traffic in
the same lane, known as weaving.
• Weaving is a consequence of having too many grade separated
junctions on a road in a short distance, where traffic wanting to
leave the grade-separated road at the next junction has to fight for
road space with traffic which has just entered from the previous
• Weaving can be alleviated by using collector/ distributor roads to
separate entering and exiting traffic.
The major advantage of cloverleaf's is that they require only one
bridge, which makes such junctions inexpensive as long as land
• A diamond interchange is a common type of road junction, used where
a freeway crosses a minor road.
• The freeway itself is grade-separated from the minor road, a bridge
being provided for one or the other. Approaching the interchange
from either direction, an off-ramp diverges only slightly from the
freeway and runs directly across the minor road, becoming an on-
ramp that returns to the freeway in similar fashion.
A typical diamond interchange
• The Diamond interchange
uses less space than most
types of freeway
interchange, & avoids the
interweaving traffic flows
that occur in interchanges
such as cloverleaf.
Thus, diamond interchanges are most effective in areas where
traffic is light and a more expensive interchange type is not
needed. But where traffic volumes are higher, the two
intersections within the interchange, often feature additional
traffic control measures such as traffic lights and extra lanes
dedicated to turning traffic.
Diamond interchange on I-787 in
watervliet, New York
partialcloverleaf interchange or parclo interchange
• A partial cloverleaf interchange or parclo interchange is a
modification of a cloverleaf interchange. It was developed by the
Ministry of Transportationas a replacement for the cloverleaf on
Highways, removing the dangerous weaving patterns and allowing
for more acceleration and deceleration space on the freeway.
• A diamond interchange has
four ramps. A cloverleaf
interchange has eight ramps,
as does a stack interchange . A
parclo is then somewhere in
between, although six is the
An Autobahn Parclo Interchange
variation in Germany.
• Various forms of parclo interchanges are used in the
Philippines' North Luzon Expressway.
• The configuration of parclo interchanges (particularly those of
the "folded diamond" type) allows for the consolidation of toll
barriers at points where onramps and offramps run alongside
• A single large barrier can serve each onramp/offramp pair
simultaneously, reducing construction and operation expenses.
Single-point urban interchange
• A single-point urban interchange (SPUI, pronounced also called a single-
point interchange (SPI) or single-point diamond interchange (SPDI), is a type
of highway interchange. The design was created in order to help move large
volumes of traffic through limited amounts of space safely and efficiently.
• A single point urban interchange is similar in form to a diamond
interchange, but has the advantage of allowing opposing left turns to
proceed simultaneously by compressing the two intersections of a diamond
into one single intersection over or under the free-flowing road.
• The most commonly cited advantages of single point urban interchanges are
improved operation efficiency and safety as well as reduced right-of-way
requirements compared to other interchange forms.
• Left turning traffic from both directions of the intersecting roadways are
able to turn simultaneously without crossing the path of the opposing left
turns. Because traffic passing through the interchange can be controlled by a
• vehicles can clear the intersection much more quickly than in a diamond
interchange (which requires two sets of traffic signals).
• The major disadvantage of single point urban interchanges over other types
of road junctions is the increased cost due to the need for a longer or wider
• A freeway-under SPUI (as in the upper diagram) requires a wider bridge
over the free-flowing road to make room for the compressed on- and off-
• However, this disadvantage poses less of a problem in cases where the
arterial, or non-freeway road already requires a very wide bridge.
A typical freeway-over SPUI.
• A roundabout is a type of circular junction in which road traffic must travel
in one direction around a central island. Signs usually direct traffic entering
the circle to slow down and give the right of way to drivers already in the
• These junctions are sometimes called modern roundabouts in order to
emphasise the distinction from older circular junction types which had
different design characteristics and rules of operation. Older designs, called
traffic circles or rotaries, are typically larger, operate at higher speeds, and
often give priority to entering traffic. In some cases, the term "traffic circle"
has been used to describe roundabouts in North America,but generally
"roundabout" is used by engineers.
• In countries where people drive on the right, the traffic flow around the
central island of a roundabout is anticlockwise (counter clockwise). In
countries where people drive on the left, the traffic flow is clockwise.
• Statistically, roundabouts are safer for drivers and pedestrians than both
traffic circles and traditional intersections Because low speeds are required
for traffic entering roundabouts they are not designed for high-speed
motorways (expressways). When such roads are redesigned to take
advantage of roundabout principles, steps are taken to reduce the speed of
traffic, such as curving the approaches.
A major signal-controlled roundabout
in central Bristol, England. Vehicles
drive on the left, and vehicles in the
roundabout are stopped by traffic lights
to allow other vehicles to enter.
• Roundabouts are safer than both
traffic circles and traditional
junctions—having 40% fewer
vehicle collisions, 80% fewer injuries
and 90% fewer serious injuries and
fatalities (according to a study of a
sampling of roundabouts in the
United States, when compared with
the junctions they replaced).
Roundabouts also reduce points of
conflict between pedestrians and
motor vehicles and are therefore
considered to be safer for them.
However, roundabouts, especially
large fast moving ones, are
unpopular with some cyclists.
Advantages of Roundabouts
• Roundabouts are safer than signal
controlled junctions, with accidents
usually occurring at a slower speed
and at a slight angle instead of right-
angle or rear end collisions at
• Roundabouts allow U-turns within
the normal flow of traffic, which are
often not possible at other forms of
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.
These interchanges are useful for highways as well as toll roads,
as they concentrate all entering and exiting traffic into a single
stretch of roadway, where toll booths can be installed. A double-
trumpet interchange version can be found where a toll road meets
another toll road or a free highway.
Trumpet interchanges are
named as such due to their
resemblance to trumpets. The
bell of a trumpet can be seen
where the terminating highway
begins to interchange with the
continuous highway, and the
resemblance to the tubing is seen
along the connecting loop
A directional T interchange uses flyover/under ramps in all
directions at a three-way interchange. A semi-directional T does the
same, but some of the splits and merges are switched to avoid ramps to
and from the passing lane.
Directional T interchanges are very efficient, but are expensive to
build compared to other three-way interchanges. They also require
three levels, which can be an eyesore for local residents.
The Rotary Grade Separator is essentially a multi-level rotary
with traffic segregation at distinct vertical levels on the basis of mode of traffic
and not direction alone. We already accept the horizontal segregation of traffic
in separate lanes based on direction and within lanes based on speed of travel.
The Rotary Grade Separator carries this idea of segregation through to a traffic
crossing. While the flyover focuses on enabling fast movement of traffic, it
ignores the pedestrians‘ difficulty in negotiation. The biggest benefit of the Rotary
Grade Separator is that it is designed around the human being - the pedestrian
and providing him safe and secure movement and access.
A bus and truck way, car way and pedestrian way have
been provided at three separate levels, irrespective of direction of traffic. The
three rotaries are also arranged concentrically, with the car way providing the
innermost ring, followed by the bus and truck way and finally the pedestrian
Bus stops are provided at points just outside the rotary. The commuter can
now alight at these points, climb down approximately 3.0metres, cross over to
any other bus stop and take a connecting change of route within 200.00metres.
For the optimum Rotary Grade
Separator, a diameter of 100M for
the outermost rotary is proposed for
It enables a
comfortable turning radius
(several times the minimum
specification) for even the
longest transport carriers.
It generates meaningful central
open spaces that can be used as
much needed green lungs in the
tarred, concreted and built-up cities
The Rotary Grade Separator, designed by Anil Laul, is not only easier to
build, it is cheaper, more flexible and more user- and environment-friendly
While the flyover focuses on the rapid movement of traffic, the RGS is
around human beings
Bus stops are located just outside the rotary.
less than half the height of a conventional flyover
For commuters who wish to switch buses near one, a flyover is a pain.
Studies show that the commuter has to walk a minimum distance of 800mto
a kilometre with an RGS the next bus stop, which will not be more than 50-
RGS can be constructed at a fraction of the cost of a flyover in less time.
The cost saving in the RGS design approach is substantial
They are more environment-friendly
RGS have very little concrete in it and do not, therefore, trap heat, unlike
"Furthermore, when a vehicle is moving up a gradient, it
burns up more fuel, thereby emitting more exhaust, which pollutes the
Trees can be grown and gardens maintained in the green patch at the
centre of a rotary
The chance of mishaps is minimised the RGS will not distort the
aesthetics of a city
Effects OF GRADE SEPERATORS
Roads with grade separation generally allow traffic to move freely, with
fewer interruptions, and at higher overall speeds; this is why speed limits are
typically higher for grade-separated roads.
In addition, less conflict between traffic movements reduces the capacity for
Motorways, though having higher average speeds, usually have much lower
accident rates per distance travelled than roads which are not grade separated.
However, grade-separated junctions are very space-intensive, complicated
and costly, due to the need for large physical structures such as tunnels, ramps
Their height can be obtrusive and this, combined with the large traffic
volumes that grade-separated roads attract, tend to make them unpopular to
nearby landowners and residents.
New grade-separated road plans can receive significant opposition from
local groups for these reasons.