CE 463 Lecture 09 Traffic Stream Variables

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CE 463Lecture 09 Traffic Stream Variables

• Civil Engineering
• Purdue University

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Outline

• Types of traffic measurements
• Microscopic traffic characteristics
• Macroscopic traffic characteristics
• Relationship between space-mean and time-mean
  speeds
• Measuring traffic characteristics with detectors
• - One-detector system
• - Two-detector system

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Types of Traffic Measurement
Aerial photography, video cameras at raised points
Stationary observer, vehicle detectors
(inductive, pneumatic, microwave, ultrasonic,
video cameras), radars
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Microscopic Characteristics
First Vehicle Trajectory
Distance x
Space Headway s
Time Headway h
Spot Speed U
Second Vehicle Trajectory
Time t
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Microscopic Characteristics

• Time headway h time measured at a spot between
  two consecutive vehicle passages
• Space headway s distance between two
  consecutive vehicles at an instant
• Speed U instantaneous speed of a vehicle at a
  spot and at an instant

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Macroscopic CharacteristicsVolume

• N number of vehicles counted at a spot during T
  seconds
• q N/T, flow rate or volume (veh/s)
• q 3600?N/T (veh/h)
• Average time headway h T/N (s), 1/h N/T

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Macroscopic CharacteristicsDensity

• N number of vehicles counted instantaneously
  along segment of L feet
• k N/L, traffic density (veh/ft)
• k 5280?N/L (veh/mi)
• Average space headway s L/N (ft), 1/ s N/L

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Macroscopic CharacteristicsMean Speeds

• Time-mean speed Ut
• Average value of speeds of vehicles passing a
  spot during some period of time (mi/h)

Space-mean speed Us Average value of speeds of
vehicles along a road at some instant (mi/h)
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Exercise

• Calculate
• volume (veh/hr),
• density (veh/mi).
• Is the space-mean speed equal to the time-mean
  speed?

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Space-mean Speed from Spot Observations

• Lets assume that N speed measurements have been
  collected over some period of time at some spot.

The space-mean speed is
The space-mean speed in the vicinity of the spot
can be approximated as follows
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Measuring Traffic Characteristics with Detectors
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Exercise
0.20 0.37 0.18
0.58 0.17 s
10.0 s

• Calculate the detector occupancy for the above
  period.

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One-detector System
Space headway sn1
Vehicle Length ln
Speed Un
Detector Length ld
Occupancy Time tnocc
Time headway hn1
Detector
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One-detector System

• Microscopic measurements

• Macroscopic measurements

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Two-detector System