10-Intersection Control ( Transportation and Traffic Engineering Dr. Sheriff El-Badawy )

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المنصورة-قسم الهندسة المدنية –معهد مصر العالي للهندسة والتكنولوجيا

Traffic Control Devices

• Broad Category of Traffic Control Devices:• Traffic Marking

• Traffic Signs

• Traffic Signals

• To be effective, a device must:1. Fulfil a specific purpose

2. Command attention

3. Convey a clear simple meaning

4. Command respect of road users

5. Give adequate time for proper response


Traffic Markings

• Longitudinal Markings:

• Yellow line delineate the separation of opposing traffic flows

• White line marks right edge of pavement

• Broken lines are permissive

• Solid lines are restrictive

• Double lines indicate maximum restriction

• Markings must be visible at night

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Longitudinal Marking

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Yellow Line = Two-way Traffic

White Line = One-way Traffic

Solid Line = No Passing

Broken Line – Pass with Caution


Traffic Flow

Traffic Flow

Two way traffic no

passing

Two way traffic pass with caution

Yellow Line


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3


Traffic Flow

Traffic Flow

White Lines

No passing - solid line

One-way trafficBroken - pass with caution


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ONE WAY – pass with caution

TWO WAY – no passing over solid lines


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Multi-Lane

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Traffic Markings• Transverse Markings:

• Placed across travel lanes

• Stop line: Solid white lines required in urban and rural areas

• Crosswalks: Number of different ways to mark crosswalks

• Parking Lanes: Not mandatory, but improves the efficiency

Stop line


Traffic Marking

• Word/ Symbol Markings

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1. Regulate traffic, movement or parking

2. Warn of potential dangers or road conditions

3. Provide information and guidance

Traffic signs have three purposes


Traffic Signs• Regulatory Signs: Convey information on specific traffic

regulation, drivers must comply

– Right-of-way series, Speed series

– Movement series (turn signs, alignment signs, exclusion signs,

one-way signs)

– Pedestrian series, Parking series, Miscellaneous

• Warning Signs: Provides information on impending condition

that may be or are hazardous

– Changing in horizontal alignment, Intersections and grades,

Advance warning for control devices, Converging traffic lanes,

Narrow roadways, Changes in highway design, Roadway

surface conditions, Railroad crossings,

– Entrances and crossings

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Traffic Signs

• Guide Signs: Provides information on selecting appropriate

routes

• Utmost important for driver’s safety

• Route Markers, Destination Signs: Conventional roadways,

• Destination Signs: Motorways, Service Guide signs, Mileposts


Colors Have Meaning

Red Green

Blue Yellow

Black White

Orange Brown

Fluorescent Optic Yellow

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Shapes Have Meaning

Octagon Rectangle

Triangle

Diamond

Pentagon

Pennant

Rectangle

Circle

Crossbuck


Three Types of Signs

Regulatory

Warning

Guide

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Where must drivers stop?

Where are they located?


When must drivers yield?

Where are they located?

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SPEED LIMIT SIGNS


Speed Limits

Urban Areas

Parks

Rural Interstate

Urban Interstate

School Zones

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RR sign or RR painted

on the pavement are

advanced warning

signs.

Crossbucks are yield signs.

The train has the right-of-way!

Railroad Crossing Warnings

Flashing red lights

Bells

Gates

If you hear or see a train


Other Regulatory Signs

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ORANGE is for Construction

REMEMBER

Common Sense

Caution

Concentration


Markings Indicating Drivers are

Entering a Work Zone

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Work Zones Areas

Advance

Warning

Transition

Buffer

Work

Area

E

N

D


GUIDE SIGNS

US Route Marker

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Intersections

An intersection is the general area where two or more highways

join or cross.

It includes the areas needed for all modes of travel:

• pedestrian,

• bicycle,

• motor vehicle,

• and transit.

Intersections are an important part of a highway facility because

their design affect greatly:

• Efficiency,

• Safety,

• Cost of operation, and

• Capacity of the facility.

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Intersection Types

Grade separated

At-grade

With ramps

(Interchanges)

Without ramps

(meaning no

connection

between the

intersection

roads!)


At-Grade Intersection

Source: FHWA Flexibility in Highway Design

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2)Grade Separation without Ramps


3) Interchanges

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Interchanges

Utilize grade separation

Additional illustrations at http://www.kurumi.com/roads/interchanges/index.html


Intersection Design

• Reduce conflicts between road users

• Improve efficiency and safety

• Consider:

• Human factors

• Traffic

• Geometrics

• Economics

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Intersection Types

Basic intersections are

• Three-leg (or T),

• Four leg, and

• Multi-leg intersections.


Three-Leg Intersections

• The angle of intersection is formed by the intersecting

streets centerlines.

• The best angle is between 75 and 105 degrees.

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Four-Leg Intersections

• The angle of intersection is formed by the intersecting

streets centerlines.

• The best angle is between 75 and 105 degrees.


Multi-Leg Intersections

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Basic Types of Movements within

Intersections

• Crossing

• Merging

• Diverging

• Weaving


Crossing

• Direct Crossings: angle of skew 75 to 105 degrees,

• Oblique Crossing: angle of skew below 75 or above

105 degrees.

• Oblique skews should be voided if at all possible.

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Merging(Joining Traffic Stream)

Diverging(Separating from Traffic Stream)


Weaving

Weaving is the combined movement of both merging and

diverging movements in the same direction.

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Conflict Points at 4-Leg Intersections


Conflict Points at T Intersection

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Conflict Points


Cross Intersection Versus Roundabout

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Sight Triangle


Intersection Sight Triangle

Hidden Vehicle

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Intersections

• Intersections are a key feature of street design in four respects:

• Focus of activity - The land near intersections often contains a

concentration of travel destinations.

• Conflicting movements - Pedestrian crossings, vehicle and

bicycle turning and crossing movements are typically concentrated

at intersections.

• Traffic control - At intersections, movement of users is assigned by

traffic control devices such as yield signs, stop signs, and traffic

signals. For this intersections are often major cause of delay, but

helps to organize traffic and reduce the potential for conflicts.

• Capacity - In many cases, traffic control at intersections limits the

capacity of the intersecting roadways which is defined as the

number of vehicles that can be accommodated within a given period

of time.

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Intersection: Key Elements


Key Elements (1)• The major street is typically the intersecting street with greater traffic

volume, larger cross-section, and higher functional class. The minor

street is the intersecting street likely to have less traffic volume,

smaller cross-section and lower functional classification than the

major street.

• Intersection legs are those segments of roadway adjacent to the

intersection. The leg used by traffic approaching the intersection is the

approach leg, and that used by traffic leaving is the departure leg.

• Sidewalks, crosswalks and pedestrian curb cut ramps are

considered to be within the intersection. The pavement corner is the

curve connecting the edges of pavement of the intersecting streets.

• The angle of intersection is formed by the intersecting streets’

centrelines. If the angle departs significantly from right angles, the

intersection is called skewed intersection

• Auxiliary lanes are lanes added at the intersection, usually to

accommodate turning motor vehicles. They may also be used to add through

lanes through an intersection.

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Key Elements (2)

• Channelizing and divisional islands may be added to an intersection

to help delineate the area in which vehicles can operate, and to

separate conflicting movements. Islands can also provide for

pedestrian refuge.

• A turning roadway is a short segment of roadway for a right turn,

delineated by channelizing islands. Turning roadways are used where

right-turn volumes are very high, or where skewed intersections would

otherwise create a very large pavement area.

• Traffic control devices assign right of way, to both motorized and

non-motorized traffic and include traffic signals, pavement markings,

STOP signs, YIELD signs, pedestrian signal heads and other devices

•


Intersections

The functional area of an

intersection includes any

auxiliary lanes and their

associated channelization.

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Elements of the Functional Area of an Intersection

The functional area on the approach to an intersection or

driveway consists of three basic elements:

(1) Perception-reaction distance,

(2) Maneuver distance, and

(3) Queue-storage distance.

Functional of Impact Length

Maneuver DistanceDecision

Distance

Stopping Queue

or Storage

length

Begin

DecelerationBegin

Perception Reaction

Deceleration

Completed


At-Grade Intersection Design

Objectives and Considerations

Need to meet two conflicting objectives:

Minimize the severity of potential conflicts among different

streams of traffic and between pedestrians and turning vehicles.

Provide for the smooth flow of traffic across the intersection

Operating

characteristics of

both the vehicles

and pedestrians

Adequate pavement

width and approach

sight distances must be

provided.

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BASIC DESIGN PRINCPLES


T - Intersections

• Suitable for minor or local roads.

• May be used when minor roads

intersect important highways with

an intersection angle less than 30

degrees from the normal.

• Also suitable for use in rural two-

lane highways that carry light

traffic.

• At locations with higher speeds

and turning volumes, which

increase the potential of rear-end

collisions between through

vehicles and turning vehicles.

• Usually an additional area of

surfacing or flaring is provided.

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T - Intersections

Provide auxiliary lane:

in cases where left-turn

volume from a through

road onto a minor road

is sufficiently high but

does not require a

separate left-turn lane.

Auxiliary

Lane

• Channelized T intersection

in which the two-lane

through road has been

converted into a divided

highway through the

intersection.

• Intersection of this type

probably will be signalized.


Channelization: Objectives

• Direct the paths of

vehicles

• Control the merging,

diverging, and crossing

angle of vehicles

• Reduce the amount of

paved area

• Provide a clear indication

of the proper path for

different movements

• Give priority to the

predominant movements

• Provide pedestrian refuge

• Provide separate storage

lanes for turning vehicles

• Provide space for traffic

control devices for

visibility

• Control prohibited turns

• Separate different traffic

movements at signalized

intersections with multiple-

phase signals

• Restrict the speeds of

vehicles

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4-Leg Intersections

• Mainly used at locations

where minor or local roads

cross.

• it also can be used where a

minor road crosses a major

highway.

• In these cases, the turning

volumes are usually low and

the roads intersect at an

angle that is not greater

than 30 degrees from the

normal.

Simple Unchannelized Intersection


4-Leg Intersections

• Right-turning roadways are

provided when right-turning

movements are frequent.

• Also common where

pedestrians are present.

Right

Turning

Roadways

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4-Leg Intersections

Suitable for:

• A two lane highway

• Not a minor crossroad.

• Carries moderate volumes at high speeds

• or operates near capacity.


4-Leg Intersections (Channelized)

Suitable for:

• Four lane approaches

• Carrying high through volumes and high turning volumes.

• Usually signalized

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Multi-Leg Intersections

• Multi-leg intersections have five or more approaches.

• Whenever possible, this type of intersection should be avoided.


Multi-Leg IntersectionsIn order to remove some of the conflicting movements and increase

safety and operation one or more of the legs are realigned.

Note: the distances between the aligned

intersections should be great enough to

allow for the independent operation of

each intersection.

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Intersection Control


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Intersection Control

• The purpose of traffic control is to assign the

right of way to drivers.

• Thus to facilitate highway safety by ensuring

the orderly and predictable movement of all

traffic on highways.

• Control may be achieved by:

• Traffic signals,

• Signs,

• Markings


Traffic Signals

• One of the most effective ways of controlling

traffic at an intersection.

• Improve overall safety by eliminating many

conflicts.

• Decrease average travel time and increase

capacity through an intersection

• Equalize the quality of service for all or most

traffic streams

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Traffic Signal

• Separate individual movements in time

rather than in space


Terminology & Definitions

• Green time: The time period in which the traffic signal has the green

indication

• Red time: The time period in which the traffic signal has the red indication

• Yellow time: The time period in which the traffic signal has the yellow

indication

• Cycle: One complete rotation or sequence of all signal indications

• Cycle time (or cycle length): The total time for the signal to complete

one sequence of signal indication.

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• Lost Time: A portion of green or yellow time which is not utilised by traffic flow movements in a cycle.

• Start-up lost time (l1): At the beginning of each green indication as the first few cars in a standing queue experience start-up delays. This delay is measured as start-up lost time.

• The clearance lost time (all-red): It is estimated by the amount of the yellow time not used by vehicles.

• Phase: That part of a cycle allocated to a stream of traffic or a combination of two or more streams of traffic having the right-of-way simultaneously during one or more intervals.

• Effective green time (gi): The effective green time, for a phase, is the time during which vehicles are actually discharging through the intersection,

Gi = actual green time, Yi = yellow time, tL = lost times for a phase i.

Terminology (Cont’d)


Terminology (Cont’d)

• All-red interval. The display time of a red

indication for all approaches.

• It is sometimes used as a phase exclusively

for pedestrian crossing or to allow vehicles

and pedestrians to clear very large

intersections before opposing approaches are

given the green indication.

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Two Phase Signal System


Traffic Signal Times9

0

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Saturation Flow

Number of vehicles that would pass through

the intersection during an entire hour of green

G

Time (sec)

Flo

w R

ate

(vp

hpl)

Y

Saturation Flow S

Given h, S=?

Effective Green Time

Effective Green G’

G

Time (sec)

Flo

w R

ate

(vp

hgpl)

Y

Saturation Flow

Time during which the flow is assumed to take

place at saturation flow

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Lost Time

Effective Green G’

G

Time (sec)

Flo

w R

ate

(vp

hgpl)

Y

Saturation Flow

Time during which no flow takes

place

Lost Time I2

Lost Time I1

21 llGYG


Discharge Headways

• Consider N vehicles discharging from the intersection

when a green indication is received.

• The first discharge headway is the time between the

initiation of the green indication and the rear wheels of

the first vehicle to cross over the stop line.

• The Nth discharge headway (N>1) is the time between

the rear wheels of the N-1th and Nth vehicles crossing

over the stop line.

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Discharge Headways

• The headway begins to level off with 4 or 5th vehicle.

• The level headway = saturation headway

• Start-up lost time can be expressed as: 1 ( )l e i


Saturation flow rate

In a given lane, if every vehicle consumes an

average of h seconds of green time, and if the signal

continues to be uninterruptedly green, then S vph

could enter the intersection where S is the

saturation flow rate (vehicles per hour of green time

per lane) given by

3600S

h

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Intersection Capacity

The maximum hourly rate at which persons or vehicles can be reasonably expected to traverse a point or uniform segment of a lane or roadway during a given time period under prevailing traffic and roadway conditions.

(HCM 2000)

•Unlike saturation flow, capacity considers traffic signal conditions

The formula for calculating capacity (c) is,

c = (g/C) · s · NWhere:

c = capacity (pcu/hour)g = Effective green time for the phase in question (sec)C = Cycle length (sec)N = no. of laness = Saturation flow rate (pcu/hour)

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Phase Design Steps

• Determination of cycle length (time)

• If tLi is the start-up lost time for a phase i, then the total start-up lost time per cycle,

• where N is the number of phases.

• If start-up lost time is same for all phases, then the total start-up lost time for cycle, L = N tL.

• If C is the cycle length in seconds, then the number of cycles per hour = 3600/C.

• The total lost time per hour = 3600L/C.

• Substituting in L = N tL, then total lost time per hour will be as follows:

• L =

N

i LtL1

C

tN L..3600

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• The total effective green time Tg equal one hour

minus the total lost time in an hour as follows:

• Tg=

• Critical lane volume that can be accommodated

per hour is given by Vc = Tg/h

• Vc=

• Then, C= i

c

L

s

V

tN

1

.


Green splitting

• Total effective green time, Tg

• Actual green time , Gi

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• Example:

• The phase diagram with flow values of an

intersection with two phases is shown in the

following figure. The lost time and yellow time for

the first phase is 2.5 and 3 seconds,

respectively. For the second phase the lost time

and yellow time are 3.5 and 4 seconds,

respectively. If the cycle time is 120 seconds,

find the green time allocated for the two phases.


• Answer:

• Critical lane volume for the first phase, Vc1 = 1000 veh/hr.

• For the second phase, Vc2 = 600 veh/hr.

• The sum of the critical lane volumes = 1000+600 = 1600 veh/hr.

• Effective green time can be computed from equation as

• Tg =120-(2.5-3.5)= 114 seconds.

• Green time for the first phase, g1 and second phase can be computed from:

• ondsg sec25.711141600

10001

ondsg sec75.421141600

6002

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• Actual green time can be determined from:

• First phase, G1 = 71.25-3+2.5 = 70.75 seconds.

• Second phase, G2 = 42.75-4+3.5 = 42.25

seconds.

• The phase diagram can be drawn in the

following figure.


Right Turn on Red

• Drivers in all 50 states are allowed to complete

a right turn on red but drivers must

come to a complete stop, yield to any and all

traffic and make sure there is no sign

prohibiting the turn, before completing a right

on red.

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Left Turn on Red?

• There is no left turn on red.


Multi-use Traffic Lanes

• In larger cities, the use of reversible lanes

increases the capacity of the existing streets.

• Center lanes are reversed to allow heavy

traffic, in one direction or the other, to have

more lanes.

• These lanes are marked with a green arrow or

a red X above the lane.

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