FHWA Traffic Analysis Tools Measures of Effectiveness

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FHWA Traffic Analysis Tools Measures of
Effectiveness

• Richard Dowling
• July 28, 2006

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The Problem

• The HCM and traffic simulation tools rarely give
  same performance results.
• No guidelines on how to interpret performance
  results.

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Project Objectives

• To develop information and guidance on
• Which MOEs should be produced,
• How they should be interpreted, and
• How they should be defined and calculated in
  traffic analysis tools

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Approach

• Review current practice.
• Review how MOEs computed from field data.
• Review how MOEs are defined and computed within
  traffic simulation models and the HCM.
• Develop innovative approach to interpret MOEs.
• Demonstrate approach in a case study.

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Status

• Draft report being reviewed by expert panel.

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Microsim MOEs

• Currently
• Travel time, speed, delay, queues
• Critique
• Computed differently in different software
• Breakdown at high congestion levels
• Hard to interpret

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Examples of Computation Differences

• Mean speed
• Definition of Queue
• Queue length

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1. Mean Speed on Arterial Link
Time
Signal
Analysis Period
Vehicle Trajectories
Distance
Link Length
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Mean Speed - SimTraffic
Time
SpeedVMT/VHT
Distance
10
Mean Speed - CORSIM
SpeedVMT/VHT
Time
11
All traces used for VHT. Only traces 1-11 for VMT.
Distance
1
11
Mean Speed - AimSun
Time
SpeedVMT/(VHT)
0
ExitTime
EntryTime
Distance
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Recommendation

• Adopt analysis domain that is limited to only
  some of the vehicles in the simulation period.
• Analysis period is specified by user.
• Simulation period is amount of time required to
  clear all vehicles entering (or attempting to
  enter) link during analysis period.

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(No Transcript)
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2. Definition of Queue

• HCM vehicle waiting to be served.
• SimTraf speed lt 11 km/h
• Corsim speed lt 3-9 km/h
• Vissim user defined, (def. lt 5 km/h)
• Paramics user defined, (def. lt 7 km/h)
• Aimsun user defined.

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Signal Example
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Queue Overflows Link
HCM, Synchro, Vissim(2)
Aimsun, Corsim, Vissim Paramics, SimTraf
Distance
Link Length
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Queue Does Not Clear
HCM/Synchro Max.Back.Queue
Simulation Max.Back.Queue
Simulation Max Queue
Analysis Period
Distance
Link Length
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Recommendation

• Adopt standard maximum speed and minimum spacing
  for queues in US.
• Adopt queue performance measures that identify
  overflow potential when queue exceeds 90 of
  storage capacity.
• Do not rely upon simulation reported queue
  lengths when queue exceeds 90 of storage
  capacity.

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Breakdown of Some MOEs

• Under parking lot conditions
• Speed goes to zero.
• Density goes to jam density.
• Stops go to zero.
• Variance goes to zero.

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Recommendation

• Place greater reliance on travel time and delay
  performance measures that do not flatten out
  under extreme congestion (parking lot) conditions.

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MOEs for Decision Makers

• Indices that have obvious good and bad values.
• Reference each index to a desired state
• Continue to be meaningful under extreme LOS F
  congested conditions.

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Recommended MOEs

• Travel Time Index
• Percent Delay
• Buffer Index
• Intersection Blockages (, Hours)
• LOS F Segments (, Hours)
• Fuel Consumption Ratio
• Air Pollution (NOx) Emissions Ratio

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Travel Time Index

• Ratio of actual to free-flow travel time.
• For freeways TTI gt 1.5 means over capacity.
• For signalized arterials, TTI gt 2.5 means over
  capacity and/or poor signal coordination.

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TTIs at Capacity Uninterrupted Flow Facilities
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TTI at Capacity Uncoordinated Arterials
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TTI at Capacity Coordinated Arterials
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Percent Delay

• Ratio of mean delay to mean trip time at
  free-flow speeds.

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Buffer Index

• Excess travel time that must be budgeted to
  ensure 95 probability of arriving on-time
  divided by the mean trip time.
• Values of 25 to 33 typical of urban commutes.

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Fuel Consumption Index

• Ratio of actual fuel consumed to hypothetical
  fuel consumed at fuel efficient travel speeds, no
  stops.
• Values near 1.00 indicate near optimal operations.

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Air Pollution Index

• Ratio of nitrous oxides (NOx) emissions to
  emissions at minimal polluting travel speed (no
  stops).
• Values of near 1.00 indicate near optimal
  operation.