Traffic Flow Characteristics

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Traffic Flow Characteristics
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Basic Measures of Traffic Flow

• Speed
• Flow Rate
• Density
• Spacing and Headway
• Lane occupancy
• Clearance and gap

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Speed

• Rate of motion
• Individual speed

• Time mean speed (spot speed)
• Arithmetic mean of all instantaneous vehicle
  speeds at a given spot on a roadway section

Space mean speed -The mean travel speed of
vehicles traversing a roadway segment of a known
distance (L). Harmonic mean -More useful for
traffic applications
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Speed

• Space mean speed is always less than time mean
  speed

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Individual Speed
(1)
Spot Speed
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Time Mean Speed
Mile post
Observation Period
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Space Mean Speed
Observation Distance
Observation Period
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Flow Rate (q)

• The number of vehicles (n) passing some
  designated roadway point in a given time interval
  (t)
• Units are typically vehicles/hour

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Density (k)

• Number of vehicles occupying a given length of
  roadway
• Typically measured as vehicles per mile (vpm), or
  vehicles
• per mile per lane (vpmpl)

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Density (k)
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Density (k)
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Spacing (s)

• Front bumper to front bumper distance between
  successive vehicles

S1-2
S2-3
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Headway (h)

• Time between successive vehicles passing a fixed
  point

T0 sec
T3sec
h1-23sec
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Spacing and Headway
spacing
headway
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Spacing and Headway
What are the individual headways and the average
headway measured at location A during the 25 sec
period?
A
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Spacing and Headway
What are the individual headways and the average
headway measured at location A during the 25 sec
period?
A
h1-2
h2-3
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Lane Occupancy

• Ratio of roadway occupied by vehicles

L1
L2
L3
D
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Clearance (c) and Gap (g)

• Front bumper to back bumper distance and time

Clearance (ft) or Gap (sec)
Spacing (ft) or headway (sec)