The primary goal of the research was to examine the system-wide impacts of early-deployment Cooperative Adaptive Cruise Control (CACC) dedicated lane strategies on roadway performance based on a variety of external factors such as overall demand, market penetration of the equipped vehicles, dedicated lane use strategy and CACC operational parameters such as inter-vehicle headway, critical gap for leading and following vehicles, and critical speed differential for leading and following vehicles.
CACC technology uses a combination of sensors and vehicle-to-vehicle and infrastructure-to-vehicle communications to enable vehicles to automatically adjust their speed to the speed of a preceding vehicles in the same lane. Dynamic Speed Harmonization (DSH) systems, typically controlled by management centers, dynamically adjust vehicle speeds in response to downstream congestion in an effort to improve throughput and reduce congestion shockwaves that can contribute to secondary crashes.
A simulation based approach was used to assess the operational and safety impacts of CACC and DSH at different levels of market penetration. The simulation test bed represented a 13-mile section of I-66 in Northern Virginia. The facility included one peak-hour peak-direction HOV 2+ lane, two to three general purpose lanes, and a shoulder lane made available during peak periods.
The simulation used a modified Intelligent Driver Model (IDM) enhanced with lane-change logic to assess the impacts of a CACC dedicated lane on traffic flow. DSH impacts were assessed using simulation software (VISSIM).
The figure below extracted from the source report shows traffic speed profiles on I-66 (WB) and the potential for reduced traffic shockwaves when DSH is combined with CACC at market penetration rate of 25 percent on exclusive dedicated lanes.
When compared to the base-case, the DSH application reduced the shockwaves by dispersing concentrations of slow-downs on the network. Additionally, when CACC was implemented, there were virtually no more slow-downs in the network and there were significant improvements in environmental savings. For example, in the DSH-only case (use case "b") increased the fuel consumption by nearly 20 percent over the base-case, whereas the combination of DSH and CACC (use case "c") reduced the fuel consumption by over 16 percent over the base-case.