TRAFFIC ENGINEERING

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TRAFFIC ENGINEERING
VOLUME
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TRAFFIC STREAM PARAMETERS

• Volume or rate of flow
• Speed
• Density

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TRAFFIC COUNTING

• Most basic measurement unit in traffic
  engineering is counting
• Count provides estimates of
• Volume and flow rate
• Demand
• Capacity

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TRAFFIC COUNTING

• VOLUME
• number of vehicles, pedestrians, etc. passing a
  point during a specific period of time
• for vehicles, usually expressed as veh/hour (vph)
  or veh/hour/lane (vphpl)

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TRAFFIC COUNTING

• DEMAND
• number of vehicles, pedestrians, etc. that desire
  to travel past a point during a specific period
• Frequently higher than volume during certain peak
  times
• Trips are diverted or not made when there are
  constraints in the system
• difficult to measure actual demand because
  capacity constrains the demand

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TRAFFIC COUNTING

• CAPACITY
• maximum number of vehicles that can pass a point
  during a specific period of time
• A characteristic of the roadway or facility
• Maximum number of vehicles that can reasonably be
  expected to be served in the given time period

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CHARACTERISTICS OF TRAFFIC FLOW

• Highly variable
• Time of day
• Day of week
• Season
• Road characteristics
• Direction

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Traffic Typically Peaks twice per day
Source www.ecn.purdue.edu/darcy
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VOLUME STUDIES

• AADT Annual average daily traffic
• ADT Average daily traffic
• DHV Design Hourly Volume
• K Design Hourly Volume Ratio
• D Directional Distribution Ratio
• DDHV Directional Distribution Hourly Volume

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Annual Average Daily Traffic

• An average 24 hours traffic volume over a full
  year (365 days)
• Total traffic during the year divided by 365
• Measure for a full year (counted for 365 days)

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Annual Average Daily Traffic
? Not directly used in highway design (road
design standard), BUT ? an intermediate value
to calculate average daily traffic (ADT) or
design hourly volume (DHV)
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AADT DATA HELPS TO

• Estimate highway revenues
• Establish overall volume trends
• Establish annual accident rates
• Analyze benefits of road improvements

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AVERAGE DAILY TRAFFIC

• An average 24 hours traffic volume for some
  period of time less than a year
• Measure for 6 months, a season, a month or a day
• Average volume per day

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APPLICATION OF ADT
ROAD CLASSIFICATION

• Roads are divided into two groups by area, i.e.
  Rural or Urban, depending on area of service,
  land use, population and travel patterns.
• Urban area defined as area with population more
  than 1,000 buildings, or 5,000 population.
• Rural area can be regarded as areas other than
  urban areas.

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Road categories

• Roads are classified by function, character and
  service.

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Design standard
(JKR Standard Classification)
R6 / U6 Rural and Urban Expressway Provides the
highest geometric design standard. They serve
long trips with high speed, comfort and safety.
Its always designed with divided carriageway and
full access control. R5 / U5 Highway, Primary
road and Arterial Provides high geometric
standard and serve long to intermediate trip
length with high to medium travelling speeds.
Its usually with partial access control.
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R4 / U4 Primary Road, Secondary Road, Minor
Arterial Major Collector Provides
medium geometric standard and serve intermediate
trip length and medium travelling speeds. Its
usually with partial access control. R3 /
U3 Secondary Road, Collector or Local Major
Street. Provides low geometric standard and serve
mainly local traffic. Partial or no access
control. R2 / U2 Minor Road, Local
Street Provides low geometric standard for two
ways flow and serve local traffic with low
commercial traffic (No access control).
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R1 / U1 Local Street Provides low geometric
standard and low traffic where two ways flow is
low (No access control).
Table 1.3 Design Standard
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GB Question

• What projection period does AASHTO suggest as a
  design period for traffic volume?

• Range is 15 to 24 years
• 20 years

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TRAFFIC PREDICTION

• Physical life expectancies
• right-of-way and grading - 100 years,
• minor drains - 50 years,
• bridges - 25-100 years,
• pavement - 10-30 years.
• The time horizon of traffic projection depends on
  uncertainty of prediction. A period of 20 years
  is widely used. Sometimes this period is 5-10
  years if predicting traffic is difficult.

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Example 1.1 Determining Design standard and
Access control Given that Group
area Urban Road category Arterial Traffic
volume (2002) 8570 veh / day Annual traffic
growth 4 per annum Design period 20
years Projected Annual Daily Traffic, AADTn
AADT0 (1r) n Where, n end of design life
in years r growth rate AADT0 current
AADT
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Solution Traffic volume on road completion
(2005) 8,570 (1 0.04) 3 9,640 veh /
day   Traffic volume at end of design life
(2025) 9,640 (1 0.04) 20 21,123 veh
/ day From table 1.3, Design standard U5
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DESIGN HOURLY VOLUME

• ? future hourly volume (both directions) used for
  design
• ? Considerable variation in hourly traffic
  volumes over a year

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DESIGN HOURLY VOLUME

• Which hour?
• 1) Average hourly volume inadequate design
• 2) Maximum peak hour not economical
• 3) Hourly volume used for design should not be
  exceeded very often or by very much
• 4) Usually use 30th highest hourly volume of the
  year
• 5) Tends to be constant year to year

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DESIGN HOURLY VOLUME

• Typically, 30HHV is equal to
• 15 of AADT in rural areas
• 8-12 in urban areas.

Why 30th HHV? 1) Compromise to high is
wasteful, too low poor operation
2) Approximately median weekly peak hour volume
(top highest week peak hours)
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Design Hourly Volume
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DESIGN HOURLY VOLUME RATIO
1) K-factor represents proportion of AADT that
occurs in the 30th HHV 2) K-factor __DHV x
100 AADT 3) K 8 to
12 urban, 12 to 18 rural
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DESIGN HOURLY VOLUME RATIO
If AADT is 3500 vpd and the 30th highest hourly
volume for the year is 420 vph what is the
K-factor for that facility?

• K-factor DHV x 100
• AADT
• K-factor 420 x 100 12
• 3500

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Question Whats the impact of choosing
different K factor for design?
If AADT is 3500 vpd, how will the design volume
differ for k-factor 8 vs. 12?
DHV K-factor x AADT
100 DHVk8 8 x 3500 280 vph
100 DHVk12 12 x 3500 420
vph 100 (different is 140 veh)

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DIRECTIONAL DISTRIBUTION RATIO

• one way volume in peak direction (expressed as a
  percentage of two-way traffic)
• 55 to 80
• Use to find Directional Design Hourly Volume
  (DDHV)
• DDHV K x AADT x D
• DHV x D

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DIRECTIONAL DISTRIBUTION RATIO
EXAMPLE
If D 6040, what is directional distribution of
traffic for previous example (Design hourly
volume 420 veh/hr)? Directional Design Hourly
Volume (DDHV) 0.6 x
420 252 veh/hr Notice we use 0.6 not 0.4!!
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DIRECTIONAL DESIGN HOURLY VOLUME

• Total hourly traffic in both directions is used
  to design two-lane roads.
• Directional traffic distribution is consistent
  day by day over years.
• Measured values can be used for the year of
  prediction.

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EXAMPLE

• Assuming a 60/40 directional split (60 in the
  peak direction) and a design hour factor (K) is
  12, what is the projected directional design
  hour volume on this extension? If AADT is 20,000
  vec/day
• DHV 2400 vec / hr / dir
• DDHV 1440 vec / hr / dir

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KNOWLEDGE

• Typically Composition vehicle,
• Cars 80
• Vans 10
• Medium lorries 4
• Heavy lorries / buses 6
• From arahan teknik 8/86, Vehicle x DDHV x pcu
• Table 3.3
• Cars 0.8 x 1440 x 1 1152
• Vans 0.1 x 1440 x 2 288
• Medium lorries 0.04 x 1440 x 2.5 144
• Heavy lorries / buses 0.06 x 1440 x 3 259
• Total 1843 pcu / hr/ lane

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KNOWLEDGE

• Ideal capacity 2000 pcu/hr/lane (with median)
• If lane width 3.5 m/lane
• From Arahan Teknik 8/86- HCM, fw 0.85
• If location is terrain and heavy 6
• From Arahan Teknik 8/86- fhv 0.93
• Capacity of road to be design 2000 x 0.85 x
  0.93
• 1581 pcu/nr/ln

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KNOWLEDGE

• If JKR want to have Level Of Service C
• From table 3.5 (Arahan Teknik 8/86)
• V / C ratio 0.7
• So, service flow 0.7 x 1581
• 1107 pcu/hr/ln
• No of lane required 1843 / 1107
• 1.665
• 2 lane required

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SPEED AND DENSITY
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SPEED

• Distance travelled by a vehicle during a unit of
  time.
• Expressed in kilometres / hour
• 2 important speed parameters
• a) Free-flow speed (uf)
• b) Optimum speed (uo)

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SPEED

• Free-flow speed (uf) is the absolute maximum
  speed that is attained when the flow approaches
  zero (i.e., only one vehicle exists on the
  highway).
• Optimum speed (uo) , on the other hand, is the
  speed of the traffic stream under maximum flow
  conditions (i.e., capacity conditions).

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SPEED

• Operating Speed
• Running Speed
• Design Speed
• Headway / Spacing

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OPERATION SPEED

• The speed at which drivers are observed
  operating their vehicles during free - flow
  conditions
• Design speed limit (85th free-flow speed
  percentile maximum reasonable speed)
• Related to spot speed study

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RUNNING SPEED

• Total time during which the vehicle is in motion
  while traversing a given highway segment
• average speed when traffic is reasonably
  continuous
• length of highway section
• running time

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DESIGN SPEED

• Selected speed used to determine the various
  geometric design features of the roadway

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Values represent the minimum acceptable design
speeds for the various conditions of terrain and
traffic volumes associated with new or
reconstructed highway facilities
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HEADWAY / SPACING

• Time gap between 2 vehicles in the traffic stream
• Distance gap between 2 vehicles in the traffic
  stream
• Apply to individual pairs of vehicle within the
  traffic stream

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HEADWAY / SPACING

• Useful for many traffic analysis proposes
• a) Obtained for every pair of vehicle, can
  
• collected in short period (example 1000
  
• vehicle in 15 minutes).
• b) Get flow, density and average speed
• c) Allow various vehicle type in the traffic
• stream to be isolated

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DENSITY

• Number of vehicles presents over a length of a
  highway at a given instant in time
• Unit vehicle / kilometre
• Difficult to measure
• 2 important density parameters
• 1) Jam density (kj)
• 2) Optimum density (ko)

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DENSITY

• Jam density (kj) occurs under extreme congestion
  conditions when the flow and speed of the traffic
  stream approach zero.
• Optimum density (ko) occurs under maximum flow
  conditions.