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132 This project expanded the current spectrum of understanding with respect to dedicating lanes to CAVs, but unknowns remain that will require further research to understand how different applications impact managed lane facilities. This chapter suggests specific research directions that may need to be undertaken in the near future. 9.1 Granular Energy and Environmental Impact Assessment The energy and environmental measures used in this project are simplified microscopic models that do not consider vehicle-specific calibration parameters in the modeled regions. To build on the results of this research, future efforts could include a more granular investigation of performance measures. The assessment of environmental impacts could be expanded with the adoption of MOVES to estimate the energy consumption and criteria pollutant emissions based on vehicle trajectories. 9.2 Expansion for More Test Scenarios The scenarios assessed in this research were selected to maximize the opportunity to address the uncertainty of dedicating lanes for CAVs on freeways. These scenarios were chosen as a building block to address the most common question encountered about committing a lane to CAV usage. To expand the number of possible scenarios, the following scenarios could be explored: ⢠Varying speed limits; ⢠Different heavy vehicle composition in traffic flow; ⢠Increasing the number of dedicated CAV lanes; and ⢠Higher levels of market penetration on restricted access DLs. Adding these scenarios to the scenarios assessed in this research project will yield a larger number of study-based considerations and permutations. Selection of scenarios and consider- ations for further study could be prioritized based on the overall anticipated impact and facili- tation to the mass deployment of CAVs. The scenarios to be examined should be defined to address the uncharted transition from initial CAV deployment to full market penetration. 9.3 Dedicating Lanes for CAVs on Arterials Previous studies regarding CACC applications have focused primarily on freeway traffic manage- ment. The majority of past efforts to identify the operational, safety, and environmental impacts of CACC and similar CAV applications limited themselves to uninterrupted traffic flows or rel- atively high-speed facilities (i.e., facilities with design and posted speeds greater than 45 mph). C H A P T E R 9 Future Research Directions
Future Research Directions 133 A few studies, such as eco-approach and departure, glidepath, and eco-speed control, have focused on low-speed arterial facilities with various intersection control types (Hao et al. 2015, Altan et al. 2017, and Kamalanathsharma et al. 2015). None of these studies looked at dedicating arterial lanes to CAVs, however, because doing so would have posed a more complicated set of deployment challenges, especially associated with accommodating turning movements. Such research will require a closer assessment of the various types of controlled and uncontrolled traffic interruptions. The following traffic flow interruptions have been identified as the most common types: ⢠Road intersections (signalized and unsignalized); ⢠Pedestrian midblock crosswalks; ⢠Transit stops and loading/unloading zones along the curbside; ⢠On-street parking; and ⢠Driveways. Roadway geometric design along arterials also differs to support low-speed functionality and various vehicle mode operations. These geometric designs also need to be accounted for when considering the practicality and possible inclusion of dedicated CAV lanes: ⢠Dedicated bicycle lanes, and ⢠Center turn lanes (e.g., two-way left-turn-lanes). Additional research into the impacts of CAV applications on DLs on collectors and minor arterials, with and without intersections, may be warranted. 9.4 Dedicating Lanes to Connected and Automated Trucks Current standard practice allowing roadway facility owners and operators to dedicate lanes to heavy vehicles like trucks is mostly limited to truck climbing lanes. These lanes are used for short distances in certain areas to improve safety, ease congestion, and prevent delays by facilitating the passing of trucks and other slow-moving vehicles whose speeds drop due to sustained steep grades. Truck climbing lanes typically have not been designated as restricted for use by heavy vehicles, but other (lightweight) vehicles generally have avoided using them because they are occupied by slower moving vehicles. Creating a DL for connected and automated trucks could bring various operational, environ- mental, safety, economical, and societal impacts. Additional studies are warranted that could review these impacts and provide guidance to agencies who operate major trucking routes. The following scenario variations could be considered when developing an analysis framework to expand considerations for connected and automated truck DLs: ⢠Percent of heavy vehicles in traffic; ⢠Restriction to trucks with/without payloads; ⢠Parallel use with truck climbing lanes; and ⢠Mixed use between connected and automated vehicles and trucks. Macroscopic modeling of exit-to-exit automated freight movements also could help deter- mine operational and economic aspects of freight optimization. 9.5 Modeling Higher Levels of Automation The completed research task for this project included the demonstration of the CACC and DSH CV applications under the scenarios designed to address the identified research questions, which mostly fell under SAE J3016 Level 1 or Level 2 of automation (SAE International 2016b).
134 Dedicating Lanes for Priority or Exclusive Use by Connected and Automated Vehicles Meanwhile, the industry is moving fast to achieve higher levels of automation. A more near- term application of AVs, called âautomated highway driving,â is a combination of ACC and lane centering for use on freeways (SAE International 2016b). Automated highway driving does not require continuous supervision by a human driver. The traffic performance of such systems is likely to be nearly indistinguishable from the performance of the Level 1 and Level 2 systems that have been modeled in this research. Although small differences in lane changing and merging behaviors may occur, the impacts of these small variations could either improve traffic flow or make it worse, depending on the level of sophistication of the specific automation system. 9.6 Unified Definitions for CAV and Related Terminology Because the definitions of automated vehicle types are more mature, using the SAE J3016 defi- nitions as a basis for allowing and enforcing CAVs in DLs is a logical starting point. Nonetheless, it is suggested that attention to the ODD capabilities issue be further researched and considered early in the adoption cycle of any DL scenario.
