Understanding the Contributions of Operations, Technology, and Design to Meeting Highway Capacity Needs (2012)

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79 C h a p t e r 5 accounting for the effects of Nonrecurring Congestion This project provides a practical methodology for assessing the ability of various operational strategies, either singly or in combination with one another, to forestall or eliminate the need to construct additional lane miles of capacity within a transportation network. The methodology provides effec- tiveness assessments about both travel time and reliability at the link, corridor, and network levels. It is already integrated into two dynamic traffic assignment modeling procedures (including the open source program DTALite as well as DYNASMART-P) and can be integrated into others as well. Unfortunately, the usefulness of the methodology is con- strained by the fact that the modeling environments in which it operates do not currently account for crashes, weather, and other events that can cause nonrecurring congestion. Non- recurring congestion represents a significant part of the delay and frustration experienced by travelers and therefore has a substantial impact on both travel time and travel time reli- ability. Yet, traditional operational models do not account for its effects. Additionally, the overall effectiveness of many operational strategies, such as those evaluated under Capacity Project C05, is misrepresented if only the recurring congestion effects are considered. As an example, the use of narrow lanes as an operational improvement strategy will typically result in more capacity, reduced travel time, and improved reliability in an environment where incident effects are not considered. But narrow lanes might also increase the potential for crashes, thereby reducing their effectiveness if nonrecurring conges- tion effects are taken into account. As another example, ramp metering does not add much in terms of capacity, but it stabi- lizes flow, lowers the probability of breakdown during demand surges, and reduces the potential for crashes. In this case, the benefit of ramp metering will be underestimated unless non- recurring congestion effects are also considered. Therefore, a comprehensive assessment of the overall effectiveness of these operational strategies needs to account for their effects on both recurring and nonrecurring congestion. Therefore, the research team recommends additional work to (a) extend the capability of the DTA methodology to include an assessment of nonrecurring congestion effects and (b) incorporate this additional capability into at least one functional modeling environment. approach Nonrecurring events that significantly affect the travel time and reliability of a transportation system can be defined across three dimensions: 1. The event type (planned versus unplanned); 2. The event’s temporal scope (time of onset and duration); and 3. The event’s spatial scope (location specific versus areawide). Some existing DTA platforms, including both DTALite and DYNASMART-P, already allow users to specify the location, time, and duration of a location-specific planned event (e.g., a work zone) and then observe the effects this planned event has on network conditions and routing. However, unplanned events (such as a crash) and events that have areawide effects (such as the onset of a rainstorm) cannot be modeled. A method for testing the effects of both planned and unplanned events should be developed in this further effort. With respect to modeling location-specific planned events, one approach might be to identify up to five strategies for work zones, on the basis of previous SHRP 2 work (particularly Reli- ability Project L03), and then test the effects of these work zone strategies by using DTALite and/or DYNASMART-P. Additional enhancements are recommended to one or more DTA models to incorporate unplanned events such as crashes and weather events. For example, crash prediction models can be incorporated for freeway and arterial facilities based on methods such as those described in AASHTO’s Highway Safety Next Steps

80 Manual. This will result in a built-in stochastic model to pre- dict crashes, similar to the stochastic model that is already in place for bottleneck capacity. Up to five strategies might be identified and tested that affect crash rates (either upward or downward) and/or the duration of the crash effects. These tests should be conducted on a real-world regional network so that reported results can be used to fairly measure the capacity and reliability effects at the corridor and network levels. The results of SHRP 2 Capacity Project C01, as contained in the Transportation for Communities—Advancing Projects through Partnerships (TCAPP) website, provide the framework for an integrated planning and operations model. The results of SHRP 2 Capacity Project C05 and Reliability Project L05 will be referenced in TCAPP. This will effectively integrate planning- and operations-related findings from the SHRP 2 Capacity and Reliability program areas and serve as a significant step toward the implementation of the research. Suggested Work plan Incorporating the ability to simulate the effects of nonrecurring events on network performance into DYNASMART-P (or any other tool, for that matter) is not a trivial matter, particularly in a stochastic and time-sensitive environment. A work plan that is recommended for consideration involves four basic steps: 1. Develop and articulate the specific strategy/method for representing the effects of nonrecurring congestion in one or more DTA models. There are several different techniques by which this can be accomplished. Examples include the introduction of link-specific disutility functions and use of a probabilistic-based simulation process. There are also other ongoing efforts in the broad field of non- recurring congestion analysis that may serve as good spring- boards; an example of this might be some elements of the Surrogate Safety Assessment Model that is under investiga- tion and development at FHWA. A by-invitation workshop might be convened with key SHRP 2 safety contractors, FHWA representatives, and DTA experts to obtain a critical review of the research team’s proposed approach and also to receive additional input and guidance before starting the coding process. This work effort would reasonably require 3 to 4 months to complete. 2. Produce the necessary software code to implement the selected strategy/method within one or more existing DTA models. This is a straightforward process but nevertheless requires about 2 to 3 months after taking account of the need for testing and debugging. 3. Apply the enhanced DTA models to a real-world regional network under scenarios that involve selected operational strategies applied both singly and in combination. This effort is also straightforward and is expected to take about 2 to 3 months to complete. 4. Summarize the results of Step 3 and incorporate the resul- tant insights and findings into a final report. This final activity is expected to take about 2 months to complete.