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8/8/2019 KSR Transportation Traffic
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Indian Institute of Technology, Kharagpur
Transportation Engineering
Lesson 2Traffic Engineering
K. Sudhakar ReddyDepartment of Civil Engineering
IIT Kharagpur
Email: [email protected]
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Indian Institute of Technology, Kharagpur
General
Traffic Engineering deals with the knowledgerequired for evaluation and design of differenttransportation facilities such as
Mid-block sections of highways Intersections / junctions
Cycle tracks Pedestrian Pathways Parking facilities Bus Terminals
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Indian Institute of Technology, Kharagpur
General
The Transportation facilities are designed to
PERFORM satisfactorily
The Measure of Performance (MOP) may be
different for different facilities
Speed of Operation, Safety, Comfort and
Economy are the major considerations fordesign of transportation facilities
Environmental Issues also play a significant role
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General
Traffic Inputs required for design and evaluation
of Transportation facilities
Characteristics of Road User Drivers,
Pedestrians
Characteristics of Vehicles
Traffic
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Indian Institute of Technology, Kharagpur
Traffic Characteristics
Traffic flow is characterized by (a) traffic volume
(b) traffic speed and traffic density
Traffic volume is the number of vehicles (or
persons) crossing a given section per unit time
Different units of traffic volume are used
depending on the unit of time considered Yearly Volume, Daily Volume, Hourly Volume
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Indian Institute of Technology, Kharagpur
Traffic Volume
Yearly (Annual) Volume Number of vehicles per year
Average Yearly (Annual) Volume = Total number ofvehicles counted / number of years of counting
Annual Average Daily Traffic (AADT) = Total no. ofvehicles counted /365 (if the traffic is counted for all365 days)
Average Daily Traffic (ADT) = Total No of vehiclescounted / No of days of count (less than 365)
ADT is used if it is not possible to count for all 365days
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Traffic Volume
AAWT or AWT Average Annual Weekday Traffic
(AAWT) is obtained by dividing the total number ofvehicles counted during all the weekdays (excludingweekends) in a year by the total number of weekdays
in a year
Average Weekday Traffic (AWT) is obtained by dividingthe total number of vehicles counted during the
weekdays by the total number of weekdays(considering a period that is shorter than a year(
These units are relevant for locations where there issignificant difference between the week-day and week-end traffic recreational locations etc
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Indian Institute of Technology, Kharagpur
Traffic Volume
Hourly Flow rates Useful for design of most
transportation facilities (capacity analysis, LOSanalysis, intersection design, etc)
Selection of Design Hourly Volume Many agenciesuse peak hourly flow rates for the design oftransportation facilities
In the case of existing facilities, peak hours of trafficcan be observed and the condition during the peakhours can be evaluated
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Indian Institute of Technology, Kharagpur
Traffic Volume
For new facilities (which are to be created) the design
hourly volumes are estimated from average dailyvolumes (ADT or AADT) using empirical adjustmentfactors. As mentioned earlier, planning exercise yields
Daily traffic volumes which need to be converted intounits useful for different design exercises.
Practical observations have resulted in useful
information which can be used to estimate peak(design) hourly volume from daily volume (ADT orAADT)
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Traffic Volume
For new facilities (which are to be created) the design
hourly volumes are estimated from average dailyvolumes (ADT or AADT) using empirical adjustmentfactors. As mentioned earlier, planning exercise yields
Daily traffic volumes which need to be converted intounits useful for different design exercises.
Practical observations have resulted in useful
information which can be used to estimate peak(design) hourly volume from daily volume (ADT orAADT)
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Selection of Design Hourly Volume
Hourly Volume (Veh/h)
Noof
Horsduringwhich
VolumeonX
-Axisis
Exceed
ed
Point of Diminishing Returns Corresponding Hourly Volume isgenerally selected as Design Volume
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Indian Institute of Technology, Kharagpur
Traffic Volume
The point of diminishing returns is generally observed
to correspond to 15th to 30th highest hourly volumes.30th highest hourly volume is usually selected fordesign. This means that facilities designed for this
volume may have congestion in 29 hours in a year
A Design Hour Volume Factor (k) is generally used toexpress design hourly volume as proportion of the
AADT (ADT)
DHV (Design Hourly Volume) = AADT X k
Where k = proportion of AADT during the 30th (or anyother selected) highest hour
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Indian Institute of Technology, Kharagpur
Traffic Volume
Directional Design Hourly Volume (DDHV) Since
traffic flows are directional and can have significantdifferences in terms of the traffic volumes expected inboth the directions, it is necessary to select the traffic
in the busier direction Directional Distribution (Split) Factor is used for this
purpose.
DDHV = AADT X k X D
Where D = Directional Split in Peak Direction
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Indian Institute of Technology, Kharagpur
Traffic Volume
Design Directional Sub-hourly Flow Rate
= AADT.K.D/PHF
Typical values of k, D and PHF factor are obtained from
practical observations. These values are usuallyavailable for different conditions :- urban, sub-urban,rural, etc
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Indian Institute of Technology, Kharagpur
Traffic Volume
Design Directional Sub-hourly Flow Rate
= AADT.K.D/PHF
Typical values of k, D and PHF factor are obtained from
practical observations. These values are usuallyavailable for different conditions :- urban, sub-urban,rural, etc
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Traffic Volume - Measurement
Traffic volumes are measured by (a) manual and (b)
automated methods
The counting (survey) can be conducted for a shortduration or continuously
Information regarding number of motorised vehicles,non-motorised vehicles, pedestrians etc is collected
In the case of intersections, turning volumes are alsosurveyed
Vehicles are counted category-wise
Temporal variation of traffic is obtained
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Indian Institute of Technology, Kharagpur
Traffic Volume - Measurement
Manual count is laborious and can be carried out only for
shorter duration (one or two weeks or shorter period)
Number of surveyors can be selected from generalguidelines available for the number of vehicles that can
be counted by a person (trained) per hour without fatigue
Automated counting of vehicles can be done by usingdifferent sensors that can detect the presence or absenceof a vehicle at a location by the signal produced. Variousprinciples (variation in magnetic field, heat sensing,optical, variation in electrical resistively, etc) are used for
automatic counting
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Indian Institute of Technology, Kharagpur
Traffic Speed
We are interested in two types of average (mean)
speeds that provide us information about the (1)instantaneous (spot) speeds at a location over sometime period (2) condition of the traffic over some space
(length of road) Spot speed of a vehicle is its instantaneous speed
measured at a location
Spot speed can be measured by observing the timetaken by the vehicle to cover a short distance (trap
length). The trap length can be selected depending onthe method of measuring time.
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Traffic Speed
For manual method of time measurement (using stop
watches) the trap length can typically be 25 to 30m.However, if electronic and automated method of timemeasurement is used, the trap length can be very small
to make the speed measured instantaneous. Spot speed study is conducted to identify the design
speeds that can be selected for typical highway
sections
Spot (instantaneous) speed information is necessary
to examine the safety, accident proneness of alocation.
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Indian Institute of Technology, Kharagpur
Traffic Speed
Spot speed study can also be used to assess the effect
of improving the road condition (where increasedspeeds are expected) or that of a speed limit restriction(speed reduction expected). Mean spot speed obtained
before and after the activity can be compared forthis purpose.
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Indian Institute of Technology, Kharagpur
Traffic Speed Design Speed
The design speed for which all the geometric features
of the road are designed can be selected as the 98th
percentile speed (speed which is expected to beexceeded 2% of time)
Speed (km/h)%of
timeSpee
disLess
than
thatgiven
onX-axis 100%98%
98 Percentile Speed
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Indian Institute of Technology, Kharagpur
Traffic Speed
The mean of spot speeds collected at a location is
called time mean speed (TMS) as the speeds collectedover time (at a location) are averaged
Journey (Travel) Speed = Distance Travelled / Total
Travel time
Running Speed = Distance/ Travel time excluding
stoppages (if any for fuel at intersections, etc) Comparison of Journey speed with running speed
gives an idea about the extent of stopped delay on the
stretch.
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Indian Institute of Technology, Kharagpur
Traffic Speed
Free Speed Speed of the vehicle which is in a free
condition (not following other vehicles in the trafficstream, i.e. the vehicle in front of it is at such adistance the driver of this vehicle can choose the
speed he desires and governed by the road andneighboring conditions
As traffic volumes increase on the stretch, the journey
speeds decrease
Comparison of journey speed with free speed can give
us an idea about the degree of congestion
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Indian Institute of Technology, Kharagpur
Traffic Speed
Average Journey Speed Representative of the travel
condition on a stretch can be obtained by dividing thelength of the stretch by the average travel time
Average travel speed will vary with time depending on
the traffic volume, densities and the correspondingtravel times observed at different points of time.
The Average travel (journey) speed is Space meanSpeed (SMS) as it is obtained by considering the traveltimes of vehicles occupying some space at a given
time
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Indian Institute of Technology, Kharagpur
Traffic Speed
Average Journey Speed (SMS) of a stretch of road can be
obtained by identifying the vehicles that are there in thestretch and by averaging the travel times of those vehicles.Distance divided by the average travel time gives SMS.
TMS will be greater than SMS
If the spot speeds of 5 vehicles at a location on a stretch of5km length are 50, 60, 40, 60, 70 km/h, the TMS is
(50+60+40+60+70)/5 = 56km/h. If it is assumed that thevehicles cover the stretch at constant speed (equal to spotspeed), the travel times of the vehicles will be 0.1, 0.0833,
0.125, 0.0833, 0.0714 hours. Average travel time = 0.0926hours. SMS = 5/0.0926 = 53.98 km/h (< 56 km/h)
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Indian Institute of Technology, Kharagpur
Traffic Speed
The following empirically developed equation can be
used to estimate SMS from TMS (which is easy tomeasure)
SMS = TMS ((standard deviation of spot
speeds)2)/TMS
A number of procedures are available for measuring
travel times and delays using which travel speeds canbe obtained
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Indian Institute of Technology, Kharagpur
Traffic Density
Density (Concentration) is defined as the number of
vehicles occupying unit length of road (vehicles/km)
Density can be measured by conducting densityexperiment in which the times of entry into and exit
from the stretch are observed for all the vehicles. Fromthis data, the number of vehicles occupying the stretchat a given point of time can be obtained. Number of
vehicles divided by length gives us the density
Other procedures also can be used to obtain densities
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Indian Institute of Technology, Kharagpur
Time and Space Headways
Space (distance) headway is the space (distance
measured centre to centre or front bumper to frontbumper) separating vehicles on a stretch of road
Average distance headway (s), if available can be used
to obtain the density as 1000/s (vehicles /km)
Similarly, the difference between the arrival times of
successive vehicles at location is called as timeheadway. If average time headway (average of timeheadways of a number of vehicles arriving at alocation) can be obtained as t, flow rate can beobtained as 3600/t (veh/h)
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Fundamental Flow Diagrams
Flow (Q), speed (Space Mean Speed) V and Density (D)
are related as Q = V X D
It can generally observed that Speed depends ondensity. As densities increase, speeds of vehicles
decrease due to the closer proximity of other vehiclesand because of reduced availability of spaces formaneuvering
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Fundamental Flow Diagrams
Typical Speed-Density Relationship (Greenshields)
Density
Speed(SMS)
Free Speed
Jam Density
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Fundamental Flow Diagrams
Linear relationship between Speed and density will
yield Sped-Flow and Density-Flow relationships asgiven below (parabolic)
Density
Flow
Capacity
Jam Density
Speed
Flow
Capacity
FreeSpeed
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Fundamental Flow Diagrams
Capacity is the maximum flow possible (on a regular
basis) at the selected location If the relationship between Density and speed is linear,
the corresponding relationships between speed and
Flow and Density and Flow will be parabolic.
For these conditions, It can be shown that capacity
flow occurs at a density of Jam Density /2 and at aspeed of Free Speed /2.
Capacity (C) can be obtained as C = (VfXDj)/4
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Extreme Flow Conditions
Extreme Flow Conditions
Density
Speed(SMS)
Free Speed
Jam Density
HypotheticalContinuous Train
CaseRa
ce-track
Case
Parking
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gy, gp
Extreme Flow Conditions
Draw Speed-Flow and density Flow diagrams for the
three extreme flow conditions given in the previousslides