Active Transportation and Demand Management (ATDM) Analytical Methods for Urban Streets

CHAPTER 1. INTRODUCTION

BACKGROUND AND UNDERSTANDING

The Highway Capacity Manual (HCM) is one of the most widely-used transportation documents for evaluating the performance of transportation facilities. It has been developed and maintained by the Highway Capacity and Quality of Service Committee (HCQSC) of the Transportation Research Board (TRB), with funding provided by state and federal highway agencies. The HCM contains analytical and computational procedures for evaluating operational efficiency of various highway facilities (e.g., signalized intersections, urban street segments, interchange ramp terminals, freeway facilities, ramp junctions, unsignalized intersections, two-lane rural highways, multilane highways, freeway weaving sections, and basic freeway segments). Recent surveys for the National Cooperative Highway Research Program (NCHRP) Project 03-115 (Kittelson and Associates, 2013) confirmed that the signalized analysis procedures are the most heavily utilized procedures in the HCM. HCM procedures are implemented within several commercial software packages (e.g., HCS 2010™, Synchro™, Vistro™, and Transmodeler™), and likely used by thousands of traffic engineers and planners across the United States.

HCM guidelines and software tools are a vital resource for these engineers and planners. They provide a practical platform for evaluating and predicting traffic operational performance. They complement the traffic simulation tools; which are powerful enough to analyze advanced technologies, unique configurations, and larger networks. Simulation tools are not necessary for all types of traffic analyses. Their use requires more time, expertise, and resources than the HCM methods, which often provide sufficiently robust assessments and conclusions. Any traffic facilities, technologies, or control strategies not "covered" by the HCM, or that deviate substantially from cases considered in the HCM, must be analyzed through simulation or other alternative tools.

By continuously developing new guidelines and updated procedures to address new facilities and technologies, the HCQSC hopes to support congestion mitigation in a time of population growth and limited resources. One example of HCM evolution is the diverging diamond interchange (DDI); whose construction has increased significantly since the year 2010, but can only be analyzed through simulation or other tools. The DDIs have been a revelation in this country. They have substantially reduced both congestion and accidents at a relatively low cost, within existing right-of-ways. The recently published HCM 6th Edition provides new procedures for DDI analysis. Given that the HCM methods and software tools are accessible to a wide audience of engineers and planners, the recent inclusion of DDI procedures will hopefully facilitate the adoption of these congestion-fighting facilities.

The DDI example shows that the HCQSC is incentivized to develop HCM methods in a way that considers new technologies and control strategies. Recent editions of the HCM have added brand- new qualitative and quantitative content related to active transportation and demand management (ATDM) strategies, which mitigate congestion in creative and innovative ways. As stated in the HCM, "ATDM strategies are essentially real-time changes in geometry, lane assignment, or traffic control that are implemented in the timeframe of a few seconds." Sample ATDM strategies include adaptive signals, dynamic merge control, dynamic lane grouping, and speed harmonization. The imminent release of HCM-based ATDM analysis procedures is an important element of the U.S. Department of Transportation's (USDOT) technology transfer mandate; in that it enables these new technologies and control strategies to be intelligently analyzed and adopted by more cities, and by the broader professional practice community.

The HCM-based ATDM analysis methods available to date remain limited in terms of scope and applicability, as only freeway facilities can be analyzed. Furthermore, new and improved variants on the ATDM applications' algorithms and parameters are being developed through both virtual and field deployments. However, as previously noted, the signalized analysis procedures are the most heavily utilized in the Manual; yet none of the computational procedures for interrupted flow facilities contain support for ATDM strategy evaluation. The latest HCM (Transportation Research Board of the National Academies, 2016) states "there is relatively little research on the demand, capacity, speed, and delay effects of urban street ATDM strategies." Therefore new research is needed to develop HCM-compliant macroscopic relationships, targeted at urban street ATDM analysis.

To achieve the full benefits of ATDM, it is essential that the user community of traffic engineers and planners, and the policy decision makers they support, have ready tools to evaluate the benefits and operational impacts of specific projects in the locations where they are considered. Through development of analysis, modeling and simulation (AMS) testbeds, the Federal Highway Administration (FHWA) is producing the research and capability for in-depth assessment of different ATDM bundles under specific operational conditions. However, application to AMS tools is often beyond the capability and resources of communities that do not maintain state-of-the-art network simulation tools. Accordingly, the HCM provides an ideal vehicle to disseminate these capabilities at a level of analysis that most engineers and planners are familiar and comfortable with.

WORK ORDER OBJECTIVE

Building on recent studies and research inside and outside the HCQSC, this project provided new urban street ATDM methodologies, data sets, and content for the HCM. This content may subsequently be incorporated within other chapters throughout the HCM. To the extent possible the measures and data used in this work were based upon existing sources, methodologies, and projects. In other words, major new data collection was not performed for this project.

TECHNICAL APPROACH

The objective of this project was to develop user-friendly, practice-accessible procedures that enable local engineers and planners to assess the potential impact on facility and system capacity of ATDM actions. Accomplishing these objectives entails several challenges, which arise primarily from the fundamental nature of ATDM actions. As its name indicates, ATDM is about moving system operations from a static, partially-informed, supply-driven approach to the fundamental concept of taking a dynamic, informed approach that targets both supply-side actions and demand-oriented measures to optimize performance. ATDM seeks to dynamically monitor, control, and influence travel, traffic, and facility demand of the entire transportation system and over a traveler's entire trip chain. The dynamic, data-driven nature of these interventions, and the fact that their target may extend beyond the local facility level, raises several challenges for this work. These challenges include:

1. Variability in impacts (throughput, delay) of applying the same strategy at different times: Even when applied at the same location, with the same parameter settings, a range of outcomes are possible depending on actual realizations of stochastic factors.
2. System effects: because some ATDM strategies target individual traveler decisions, their impact may extend beyond the immediate location where they are applied; these interactions in the context of network interconnections make it difficult, in some situations, to limit or localize the impacts to specific facilities.
3. Context specificity and lack of transferability/scalability: the observed or simulated impacts of applying certain strategies may be dependent on unique characteristics of these locations. For instance, weather-related ATDM actions examined in the Chicago testbed may not transfer especially well to a city like Atlanta, where drivers are not accustomed to snow occurrences.

The research team planned to address these challenges by articulating a framework that recognizes the above factors, and providing practical ranges of possible outcomes instead of a single mean value. The framework would be developed through analysis of existing results developed in the five AMS testbeds (with particular focus on Chicago and Phoenix testbeds which are explicitly considering arterial-related strategies), actual field deployments (especially from the integrated corridor management (ICM) sites with applicable strategies), and stakeholder input. The framework would then be applied in conjunction with a scenario-based approach, similar to that developed in SHRP-2 L04 project to produce travel time reliability outputs using simulation models (Kim, Mahmassani, Vovsha, Stogios, & Dong, 2013) (Mahmassani, Kim, Stogios, Currie, & Vovsha, 2013).

Figure 1 summarizes the team's approach to the work order. For Task 1, the team began the project by finalizing the project work plan following an initial kickoff meeting. Then for Task 2, the team conducted a literature and state-of-the-practice review of relevant methodologies defined in the HCM. For Task 3 the team assembled a peer group of HCM and ATDM experts, who could continuously provide guidance throughout the project. Task 4 documented existing data sources, which could facilitate efficient execution of HCM-centric ATDM research. Task 5 identified candidate system performance measures and methods for assessing urban street ATDM effectiveness. In Task 6 the team conducted original research, based on insights gained from the other five tasks.

PEER REVIEW GROUP

The peer review group represented a unique project task, and will be mentioned throughout this report. In parallel with Task 2 – Review of Existing Methodologies, Task 3 of the project required establishment of a peer review group to provide input on development of the methodologies. Members of the group were drawn from the HCQSC, the HCQSC Task Force on ATDM, the FHWA, and other interested individuals. The peer group met periodically throughout the project's period of performance via web conference. The peer review group provided input on which urban street ATDM strategies should be implemented in the HCM, how best to conduct the original research, and how to meet overall project objectives.