Incorporating Travel Time Reliability into the Highway Capacity Manual (2014) / Chapter Skim
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Pages 24-36
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24 C h a p t e r 4 The SHRP 2 Project L08 conceptual analysis framework (L08 framework) for predicting travel time reliability is designed for operations analysis and planning applications in which the analyst must estimate current reliability on the basis of a limited amount of data, or predict the impacts of demand changes, operational improvements, and design concepts on reliability.
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25 A more significant difference in the two implementations of the L08 framework is the use of stochasticity within the methods. The L08 freeway method is primarily a deterministic approach that applies probabilities at the end of the process when tallying the reliability statistics.
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26 freeway segment, analysis period (15 min) , and overall facility levels.
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27 Scenario Generator The purpose of the freeway scenario generator (FSG) is to enumerate a sufficiently complete set of operational scenarios that a freeway facility may experience during the reliability reporting period (RRP)
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28 the number and duration of each incident from annual incident logs in data-rich environments. Furthermore, the research team's experience with using incident logs revealed significant underreporting of certain incident types.
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29 Thus, a maximum of 2 (start times) × 3 (locations)
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30 the last segment of the facility, also lasting 30 min. The segment CAF reflects the combination of the two events.
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31 Enhancements to the FREEVAL-RL Outputs Some of the scenarios run in FREEVAL-RL can clearly generate severe congestion. In some cases, the congestion might be more than the model can handle (e.g., multiple interacting bottlenecks)
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32 performance for the reliability reporting period. Facility travel time is the performance measure used to define reliability.
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33 The Center periodically publishes summaries from weather stations in each of 284 U.S. cities and territories (NCDC 2011a)
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34 this estimate is described in a subsequent section. For urban streets evaluation, crashes are categorized as • Crashes related to the segment; or • Crashes related to the intersection.
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35 and incidents is estimated for each analysis period in the reliability reporting period. One, two, or three of the seed numbers can be changed to generate a different set of conditions, if desired.
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36 • 10th, 80th, 85th, and 95th percentiles; and • Number of observations. In addition, the average base free-flow speed is always reported.
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