UNIT 2 (1)

AT-GRADE INTERSECTIONS

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 At the end of this lesson…
 Explain the objectives and techniques of intersection design
 Apply these objectives and techniques by sketching intersection proposals and
assessing other such proposals
 Know the types and features of traffic control devices

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 Formation of Intersections

T-Junction Skewed Crossed

Junction Intersection

Y-
Junction

An intersection at grade is formed when two or more roads

intersect or join at the same level. The basic geometric forms of
intersections are three-leg (T-junctions or Y-junctions), four-leg (4
approaches, called cross intersection when both roads continuing)
and multi-leg.

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 Raised traffic islands, safety
Widened bars, or painted islands

Types

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 Plain (unchannelised and unflared): The roads forming the intersection join each other
without any special treatment. Unchannelised intersections are used at the junctions of
minor roads on which traffic volumes are low or where minor roads meet or intersect with
major road/s.
 Flared: One or more of the joining roads many be widened on the immediate approaches to
and or departures from the intersection to provide additional width for through or turning
traffic.
 Channelised, including roundabouts: Traffic islands are used to guide or channel traffic into
clearly defined paths by means of raised traffic islands, safety bars, or painted islands to
produce more orderly and safer traffic operation. Roundabout is a special form of
channelised intersection at which traffic circulates (clockwise) around a central island.

 At each of the above types of intersection, traffic signals may be installed as traffic volumes
increase to allocate right-of-way to more streams of traffic at a time. Traffic signals may be
fixed-time, in which case phasing of the signals (allocation of time for each movement) is
predetermined, or they may be vehicle-actuated, in which case they are responsive within
certain limits to the varying demand of traffic on the various approaches.

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 Types of
Intersection
Maneouvres

Each of these manoeuvres can be

elemental or multiple:

• An elemental manoeuvre
occurs when any two one -
way, single-lane movements
interact.

• A multiple manoeuvre
occurs when more than two
one-way single-lane
movements take place.
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 Types of
Conflict at
Intersections

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Number of Potential Conflict Points (CP)
 Number of approaches to the
intersection (> approaches, > CP)
 Number of lanes on each approach (
>lanes, > CP)
 Type of signal control (> sophisticated
control, < CP)
 Extent of channelisation (>channelisation,
< CP)
 Movements permitted (>movements, > CP) 32 potential conflict points 9 potential conflict
points

8 potential conflict
points
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 Types of Intersection Control
Stop / Give Way Signalised

Grade Separation Roundabout

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 Objectives of Intersection Design
 Improve traffic flow, by increasing capacity and reducing delay
 Improve safety at all intersections by reducing the likelihood of accidents.
 However, in some situations specific objectives are considered:
 Arterial intersections
Ensure adequate capacity
Minimise delays
Provide for large vehicles
 Local roads
Discourage but allow entry of large vehicles
Discourage the entry of through traffic
Control vehicle speeds in and near intersections
 Rural intersections
Design for speed control

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 Principles of Safe Intersection Design
(Geometry and Capacity)
 Reducing the number of points of
conflict
 Separation of points of conflict,
convert multiple manoeuvres into
elemental ones.
 Giving preference to major
movements

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 Principles of Intersection Design
(Geometry and Capacity)
 Minimising area of conflict by
reducing excessive pavement areas
 Definition of paths to be followed

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 Principles of Intersection Design
(Geometry and Capacity)
 Control of speed

Vectors A and B represent speeds of vehicles A and B; Length of vector C represents their relative speed
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 Principles of Intersection Design
(Geometry and Capacity)
 Clearly identify priority for all roads and movements
 Provide adequate capacity and minimum delays and stops for priority traffic
 Minimize adverse costs and environmental effects

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 Principles of Intersection Design
(Geometry and Capacity)
 Design for appropriate traffic control devices
 Consider the spacing of intersection
 It is desirable that intersections at grade be such that
effective synchronisation of the signals can be employed.
This can significantly reduce energy use and improve the
operation. The easiest way of achieving this is to have
evenly spaced intersections

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 Principles of Intersection Design
(Geometry and Capacity)
 Provide adequate sight distance on the approaches to and within the intersection

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 Major Parameters in Intersection
Layout Design
 The intersection layout should be designed with due recognition of :
 Human characteristics
Reaction time, judgement of speed, distance, gap, walking speed, etc.
 Vehicle characteristics
Driver eye height, acceleration and deceleration
 Environmental factors
Weather, light, pavement condition, grades, site restrictions, etc.

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 Factors Affecting Intersection Design / Treatment
 Volume (peak hour) and composition of traffic on each approach and turning
movements at the intersection
 The classification of the intersecting roads
 Vehicle approach speeds
 The presence of over-dimensional vehicles
 Estimated delays to major and minor movements and to pedestrians

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 Factors Affecting Intersection Design / Treatment
 The presence of pedestrians and cyclists
 The presence of overdimensional vehicles
 The presence of public transport vehicles and their requirements

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 Factors Affecting Intersection Design / Treatment
 Site conditions
 Topography, abutting development and access to it, sight distance, alignment of the
intersecting roads, property boundaries, public utility services and drainage.
 Traffic safety and accident experience
 The demand for kerb side parking
 Compatibility with other intersections
 Available road reserve
 Installation and maintenance costs
 Effect of possible traffic growth and redistribution

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 Channelisation
 Channelisation is the general term used to describe the situation where conflicting
traffic movements are separated into specific path of travel often by means of
raised or painted traffic islands and associated signing and pavement markings.

 Channelisation involves the use of islands at intersections to guide and protect

traffic.

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 Channelisation
Role of Traffic Islands (Painted & Raised)
 Reduce the area of conflict
 Guide traffic movements
 Provide refuge for pedestrians
 Providing advance warning intersection
 Provide site for traffic devices and signs
 Enforce movement prohibition or enforce legal movement

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 Design Features for Raised Traffic Islands
 Raised islands should be at least 8m2 in area.
 Where an island has to provide for stop lines, traffic signals and pedestrian
crossing, it should be at least 6m long and 1.2-1.8m wide.
 Sight distance - provided to pavement at nose of island
 Rounded corners to ensure safe and smooth manoeuvres.
 The sides of a raised island should be offset a minimum of 0.3m from the edge of
the adjacent traffic lane when semi-mountable kerbs are used. This offset should
be increased to 0.6 m for barrier kerb and may be reduced to zero for a fully
mountable kerb.
 Islands should be arranged so that the driving path seems natural and convenient,
 There should be only one path for the same intersection movement,
 A few well -placed, large islands are better than many small islands,

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 Design Features for Raised Traffic Islands
 Adequate approach-end treatment is required to warn drivers and to permit gradual
changes in speed and path, (Chevron pavement markings are normally used to
define the true lane edge from which the traffic island kerb is offset),
 Vehicle turning templates should be used to ensure that vehicles can negotiate the
islands without mounting them.
 Adequate visibility should be provided to drivers approaching the intersection.
There should be no hidden obstructions, and all islands should be well defined and
clearly visible to approaching traffic.
 The approach end of islands, which divide traffic should be as narrow as
practicable without losing the visual impact of the island end for approaching
drivers. Normally a 1.0 m diameter nose is a satisfactory minimum but 600 mm
diameter nose may be desirable in some cases.
 Pedestrian requirements may influence island shapes and placement.

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 Design Features (for raised traffic islands)
Rounded corners At least
– safety & 8m2
smooth
manoeuvres Driving
path –
natural &
convenient

End treatment to warn

drivers and to permit
gradual changes is speed
and path
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 No hidden obstructions, all
islands should be well
defined and clearly visible
Pedestrian requirements
to approaching traffic
may influence island
shapes and placement
Turning templates
– ensure vehicles
can negotiate the
islands without
mounting them

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 Factors in Selecting Appropriate Treatment
 Direction signs and or warning signs, and or pavement markings and or other
delineation devices
 Stop or Give Way signs
 Flaring on one or more approaches to provide additional through and or turning
lanes
 Traffic islands and signs
 Roundabouts
 Traffic signals
 T-junction rule (minor traffic give way to major traffic)

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 Types of Intersection Traffic Control/Treatments

T-Intersections

TYPES

Cross-
Multi-Leg Intersections
Intersections

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 T-Intersections: Treatment of low angle intersection
 Desirable angle of intersection: 70-90.
 If the angle is less than 70 degrees (Y-type intersections), the intersection should be
modified (by changing the angle of intersection to a min. of 70 degrees) to overcome
potential problems (undesirable high speed turning movements, priority may not be
obvious, driver visibility may be restricted)
 Modified Treatment:
 Controls turn speed
 Reinforces priority for
increased safety

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 T-Intersections: Treaments
 Right turn lane treatments in urban areas

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 T-Intersections: Treaments
 Left turn lane treatments in urban areas

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 T-Intersections: Treaments
 Flaring to provide turn lanes and shelter to turning vehicles and less interruption to
through traffic. Suitable in rural areas due to high speeds.

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 T-Intersection: Treatment
 Roundabouts, appropriate when all legs of the intersection are of similar priority,
safety or capacity is a problem, low speed operation is appropriate.
 Signalisation, may be provided for capacity reasons in urban areas however it has
little effect on safety.

http://safety.fhwa.dot.gov/rsa/case_studies/fhwasa14003/appxa.cfm

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 T-Intersections: Treaments
 Seagull islands, appropriate where high capacity is required and the through road is
divided with a median (10m min.).

Can be signalised

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 Cross-Intersections
 This type is undesirable in rural areas unless treated to prevent high speed
crossing movements.
 Types of treatment:
 Staggered T-treatment
 Wide median
 Roundabouts
 Signalised

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 Cross-Intersections: Staggered
 Appropriate for two-lane two-way roads in rural areas where capacity is not a
problem.
 One or both of the minor road legs may be deviated to crate the stagger.
 Generally the stagger should be right-left to enable minor road vehicles crossing
major road to avoid queuing in the major road.
 Left-right stagger can be used when right turn auxiliary lanes are provided on the
major road.

Right-left stagger Left-right stagger

(Preferred)
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 Typical right-left
staggered-T
treatment
Advantage is little land
acquisition compared to
a roundabout

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 Typical left-right staggered-T treatment

• Advantage is that crossing traffic deals with one direction at a time

• Disadvantage is the extent of land acquisition

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 Cross Intersections: Wide Median
 Used only on intersections of divided rural roads with two-lane (or more) two-way
roads.
 The median has to be at least 18m wide, volume of traffic on minor road <50 vpd
(unlikely to approach capacity in the near future) and traffic signals or roundabouts
are inappropriate.

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 Cross-Intersections: Roundabouts
 This type is often used at cross-intersections as alternative to signalisation.
 Preferred when safety or capacity is a consideration, volume of right turning traffic
is high and the approach roads are of same importance (similar volumes).

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 Cross-Intersections: Signalised
 Traffic signals at intersections are provided to control traffic movements and reduce
the potential for accidents by time separation of potentially conflicting movements.
 Periods of time (phases) are provided in turn to the various movements and may
include two phases at minor intersections or many phases at complex intersections.
 Signalised intersections should provide straight paths across the intersection for
through vehicles for all approaches. Provide auxiliary lanes for turning vehicles
to enable them to slow and store clear of through traffic.
 Appropriate locations for traffic signals are mainly at urban intersections where:
 there is insufficient capacity for an unsignalised intersection to operate satisfactorily
 there are nearby signalised intersections which would enable signal linking
 it is desired to prevent excessive delays to minor movements
 there are right-of-way or economic restrictions that make installation of roundabouts
inappropriate.

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 Typical signalised cross-intersection

of vehicles

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 Multi-leg Intersections
 Multi-leg intersections can be treated by
 realignment
 use of roundabouts
 signalisation (substantial delays).

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 Guidance on Choice of Intersection
 Multi-leg intersections should be avoided.
 On rural roads, cross-roads and multi-leg intersections should be avoided and
clear T-junctions are favoured.
 On urban arterial roads at locations where traffic is anticipated to be controlled by
traffic signals or a roundabout, cross-road intersections are preferred.
 On local and collector roads in urban areas, T-junctions are favoured and cross-
roads should be constructed as roundabouts.

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Traffic Control Devices

46
 Functions
What are
TCDs?

Traffic
Features
Control
Devices
Delineation

Requirements
Placement

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 Types of TCD
 Traffic signals (control conflicting movements at intersections)
 Traffic islands
 Traffic signs
 Pavement markings, raised pavement markers
 Safety barriers
 Guide posts

BACK
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 Functions of TCD
 Regulate traffic (assign right of way or indicate regulations in force)
 Warn motorists of hazards or regulatory controls ahead (temporary hazard, sharp
curve)
 Guide traffic (signs informing direction, distance, identifying route, lane)

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Guide traffic,
regulate traffic

Guide traffic,
regulate traffic

BACK
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 Features of TCD

 It should command attention and respect

 It should convey a clear, simple meaning with the minimum number
of messages required to obtain the desired response from the driver.
 It should be capable of fulfilling an important need.
 It should not obscure any other traffic control devices or introduce a
hazard

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 Features of TCD

 Familiar to the driver and is associated with a situation

that would be predictable to the driver
 Operate effectively in all likely conditions, day/night,
wet/dry

 To achieve consistency with driver’s expectations, the

AS should be adopted for design, application and
location

BACK
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 Requirements

 Legibility and visibility, depends on size and layout of letters, contrast with
background, illumination and placement

 Clarity, use of standard (symbol + words)

 Shape consistent with purpose (warn, regulate, guide)

 Size, depends on 85th percentile speed, background, lateral position

 Location, no obstruction to sight distance, in non-hazardous area

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REGULATORY Function: to regulate the
movement of traffic and to
SIGNS indicate when or where a legal
requirement applies, failure to
comply with which constitutes
an offence

MOVEMENT PARKING

DIRECTION MISCELLANEOUS

PEDESTRIAN EXCLUSIVE-USE
LANE
SPEED
BICYCLE

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 WARNING Function: to warn road users
of unexpected or hazardous
SIGNS conditions on or adjacent to the
road

ALIGNMENT ROAD
OBSTACLES
INETERSECTION & RAILWAY LEVEL
JUNCTION CROSSING

ADVANCE WARNING AUXILIARY

OF TCD
ROAD
DIMENSIONS

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GUIDE SIGNS Function: to inform and advise
road users of directions,
distances, destinations, routes,
the location of services for road
users, and points of interest.

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TEMPORARY Function: to control, warn and
guide road users safely through,
SIGNS around or past work sites on
roads and footpaths and to
warn and advise of other
temporary hazardous conditions,
which could endanger road users

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 HAZARD Function: to delineate a
marked change in the direction
MARKERS of travel or to emphasize the
presence of an obstruction

BACK
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 MOVEMENT DIRECTION

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PEDESTRIAN SPEED

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 PARKING BICYCLE

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MISCELLANEOUS EXCLUSIVE LANE

SUPPLEMENTARY

BACK
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 Placement of Signs
 Signs should be placed within reasonable distance of hazard or event
= legibility distance + reaction distance

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 Reaction Distance
 Simple message (not involving a decision)
 80-250m at different speeds for urban and rural environments
 30-100m for business and residential urban districts
 Complex messages requiring decisions
 Distance equivalent to 12 sec travel time

BACK
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 Delineations
 Functions
 Control the position and movement of traffic, including definition of priority
 Identify the safe limits of the carriageway
 Regulate the direction of travel, lane changing and overtaking
 Warn of hazardous situations

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 Types of Delineations
Post-mounted delineator (PMD) Surface mounted delineator

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 Types of Delineations
Diagonal and chevron markings

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 Types of Delineations
Raised pavement markers

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 Types of Delineations
Line marking – Longitudinal, Transverse, and edge line

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 Types of Delineations
Messages on Pavement

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 Types of Delineations
Parking and loading zones marking

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 Role of Linemarking
 Reinforce priority (legally given by STOP & GIVE WAY signs)
 Mark waiting lines for traffic.
 Show lane boundaries and type of lanes.
 Guide traffic into lanes
 Show movements allowed and prohibited.
 All linemarkings should confirm to AS 1742.

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 Line Markings
 Longitudinal lines (broken and unbroken):
 Separation lines-separate opposing traffic lanes (broken)
 Barrier lines- prohibit vehicles using opposing traffic lanes (unbroken)
 Lane lines-separate lanes of traffic travelling in same direction (broken), if unbroken-to
discourage lane changing
 Continuity lines-indicates edge of through lane, which may be crossed by turning or
merging traffic (broken)
 Turn lines-indicate path for turning vehicles at major intersections (broken)
 Edge lines-are used to delineate the outer edges of the travelled way and to discourage
traffic from travelling on the shoulders and make driving safer and comfortable by
providing continuous guide at night (unbroken)
 Transverse Lines
 Stop lines-used where traffic is legally required to stop (unbroken)
 Holding lines-at Give Way signs (broken)
 Pedestrian crossing lines
 Devices that make a noise or change the ‘feel’ of the road, called ‘rumble strips’,
could be longitudinal or transverse devices.
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 Line Marking, Longitudinal

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SWINBURNE UNIVERSITY OF TECHNOLOGY 75
 Line Marking, Transverse

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 Painted and Removable Line Markings

Road tape

BACK
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 What will happen if there are no TCDs?
 Imagine a world without signage, traffic lights, traffic lines, etc.

 CHAOS???

SHARED ZONES

• Length of road where pedestrians, cyclists and cars

have shared access to the area
• Drivers must give way to pedestrians
• Speed limit nominated on sign applies to the road (or
network of roads) in the shared zone
• Applicable to streets, lanes or car parks with low traffic
volumes and high pedestrians numbers
• Appropriate street construction to make it look like it is
used by both vehicles and pedestrians

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 ~THANK YOU~

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