Ramp Metering For Maximizing Freeway Throughput Lily Elefteriadou, Ph'D' Alexandra Kondyli, Ph'D' Ca

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Ramp Metering For Maximizing Freeway
ThroughputLily Elefteriadou, Ph.D.Alexandra
Kondyli, Ph.D. CandidateUniversity of Florida
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What is Ramp Metering?
Signal at the on-ramp to restrict flow into the
freeway
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Ramp Metering

• Advantages
• Smoothes flow of traffic
• Breaks the platoons
• Increases throughput
• Increases vehicle speeds
• Reduces number of crashes
• Disadvantages
• Traffic diversion
• Equity

Minnesota Experiment at 2000 Meters were turned
off. Before and after evaluation showed that
meters reduced crashes by 21 and increased
mainline volumes by 9.
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Types of Ramp Metering Algorithms

• Pre-timed control fixed rates based on historic
  data, activated upon pre-set schedules.
• Local traffic responsive control based on local
  current measurements of speed, flow, or
  occupancy.
• Area-wide traffic responsive control coordinated
  control where metering rates are interrelated.

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Ramp Metering Applications

• Ramp meters have been installed around several
  urban areas in the US and elsewhere.
• All ramp metering strategies can improve traffic
  operations on freeways.
• Several algorithms consider queuing on the ramps
  and attempt to adjust the rates to minimize
  spillback.

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NCHRP 3-87 Proactive Ramp Management Under the
Threat of Freeway-Flow Breakdown

• Objective
• To develop or enhance existing ramp metering
  algorithms to delay or prevent freeway-flow
  breakdown, and increase throughput.

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What is Breakdown?

• Understood to mean the beginning of congestion
• But, need to define it quantitatively
• Need to be able to identify in data sets
  typically available from Traffic Management
  Centers
• Several definitions have been proposed (for
  example, speed-based and occupancy-based)

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Pilot Data Collection

• Investigate issues regarding the breakdown
  definition using field data
• Identify the steps required for data analysis
• Estimate time requirements for the full data
  collection and analysis.
• 4 months of data at 3 interchanges along the QEW,
  Canada

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Pilot Data Collection Site Characteristics
Cawthra Rd. Interchange (1)

• Freeway section along Queen Elizabeth Way (QEW),
  Ontario, Canada.
• Study section with three interchanges (Cawthra
  Rd, Hurontario St, Mississauga Rd.)
• All ramps are metered.

Hurontario St. Interchange (2)
Mississauga Rd. Interchange (3)
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Speed and Flow at A Freeway Ramp Junction
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Defining and Predicting Breakdown
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Breakdown Definition Issues

• Identification of breakdown vs. start of
  congestion location (Cawthra Rd.and Hurontario
  Str. both break down at similar times)
• Prediction of breakdown vs. start of congestion
  location.

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Breakdown Definition Not Easy to Develop and
Apply Universally

• Variability between days, sites (driver behavior,
  vehicle performance)
• Breakdown does not always occur at maximum flows
  observed (i.e., capacity) ? it rather occurs over
  a wide range of demands
• It does not always occur for the same combination
  of freeway and ramp flows
• The presence of platoons can expedite the
  breakdown

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Development of Breakdown Probability Model
Product-Limit Method (Kaplan and Meier, 1958)
Applied in lifetime data analysis
statistics. Lifetime Analysis What is the
distribution of a lifetime? What is the failure
time of technical components?
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Probability of Breakdown Model

• Product-Limit Method
• Survival function
• Breakdown probability
• function

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How can we use the probability of breakdown model
to set ramp metering rates?
Evaluation of current traffic conditions
Range of breakdown probability as a function of
freeway volume and ramp metering rate
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Example Application Seattles Bottleneck
Algorithm

• Current algorithm
• Estimates local metering rate (LMR) based on
  occupancy upstream of subject ramp
• Estimates bottleneck metering rate (BMR) based on
  system input/output
• The most restrictive of the two is implemented

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Proposed Enhancements (1) BMR Implementation
Activation
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Proposed Enhancements (2) LMR for critical ramps
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Proposed Enhancements (3) BMR Calculation
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NCHRP 3-87 Next Steps

• Data Analysis (7 metered and 2 non-metered
  corridors)
• Finalize Breakdown Probability Model (by type of
  site?)
• Evaluate Proposed Ramp Metering Algorithms Using
  Simulation
• Determine Whether Field-testing of the New
  Algorithms Can Be Undertaken
• Finalize Proposed Ramp Metering Algorithms

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Questions and Comments
Thank You!