Understanding the Contributions of Operations, Technology, and Design to Meeting Highway Capacity Needs (2012) / Chapter Skim
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1Executive Summary Continuing growth in urban travel demand will inevitably require more physical capacity in the transportation system. However, because of limited financial resources, high construction costs, environmental considerations, long timelines, and an increasingly complex regulatory process, capacity-adding projects have become actions of last resort.
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2This report summarizes the results of a capacity project undertaken through the second Strategic Highway Research Program (SHRP 2) to advance the state of practice in this area.
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3Dynamic traffic assignment (DTA) models are especially advantageous for evaluating strategy effectiveness because they provide a realistic assignment of traffic in oversaturated networks: • They recognize that drivers have varying levels of knowledge about the travel time on each of the travel paths available to them.
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4activity that randomly varies from day to day. During simulation periods when these random freeway queues are present, the queue discharge rate at active bottlenecks is also represented with random variation by modeling it as a time-correlated stochastic process.
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5of individual links and turn movements. Therefore, an enhanced model was developed that recognizes when queue lengths exceed available storage lengths at these locations and adjusts the downstream discharge rate accordingly.
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6Under stochastic capacity, the travel time experience on a single day can be dramatically affected by the underlying capacity on that particular day. (In this study, "day" is defined as any regular weekday; i.e., a day other than a weekend or a public holiday.)
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7• Queue growth and dissipation. A sliding time bar can be used along with a visual representation of the network to observe the start, growth, and dissipation of queues.
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8• Next, geometric, volume, and operational characteristics of each link in the subarea are identified and provided as inputs to the DTA model, including stochastic capacity distributions at the geometric or operational bottlenecks. Appropriate link, corridor, and/or network performance measures are also established for subsequent evaluation purposes.
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9• Another ATIS strategy in which the fraction of drivers with access to en route information (e.g., through in-vehicle navigation systems) increased from 1% to 10%; • An operational modification to the existing baseline condition in which the width of the freeway lanes and shoulder in a critical 3.1-mi (5-km)
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10 Figure ES.8 demonstrates how trade-offs for improvement strategies can be examined in terms of their impact on average travel time (expressed as minutes of travel time) and travel time reliability (expressed as the range between the 5th and 95th percentile travel times)
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11 enhance the usefulness and usability of these tools. In the meantime, they can still be effective in significantly improving investment decision making in the transportation industry.
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