According to the study, safety features onboard automated vehicles slow down the movement of traffic
“There are two significant reasons that people are interested in automated vehicles – improving passenger safety and reducing travel time,” said Ali Hajbabaie, first author of a paper on the work and an associate professor of civil, construction and environmental engineering at NC State.
“There is a lot of research showing that automated vehicles can improve safety. But our research here – which relies on computational modeling – suggests that if we want to also improve travel time, an increase in automated vehicles isn’t enough; we need vehicles that are capable of communicating with each other and with the traffic-control systems that manage traffic flow at intersections.”
For the study, the researchers used a computational model that simulates traffic conditions. The researchers accounted for four types of vehicles: human-driven vehicles (HVs); connected vehicles (CVs) – which are driven by humans, but share information with other connected vehicles and with the control system that manages traffic lights; automated vehicles (AVs); and connected automated vehicles (CAVs).
“Because of their programming, AVs are assumed to move more cautiously compared to human drivers,” Hajbabaie said in a statement. “Their safety stems, in part, from their being programmed to drive conservatively. CVs and CAVs are designed to receive information about the future state of traffic lights and adjust their speeds to avoid stopping at intersections. As a result, the movement of CVs and CAVs is expected to be smoother – and have a lower number of stops – than HVs and AVs.”
The researchers ran 57 traffic simulations to assess the impact of a host of variables on travel time through an intersection, such as looking at how traffic would be affected by various combinations of HVs, AVs, CVs and CAVs.
They found that the higher the percentage of CVs and CAVs, the greater the intersection capacity. Higher capacity also means that, on average, there are fewer vehicles waiting at a red traffic light.
“However, we found that higher percentages of AVs – which are not connected – actually slows travel times through intersections,” said Hajbabaie. “This is because those AVs are programmed to drive conservatively in order to reduce the risk of collisions. Our findings underscore the importance of incorporating connectivity into both vehicles and traffic-control systems.
“This study was conducted using a computational model, which is a limiting factor,” said Hajbabaie. “However, it’s difficult and expensive to assemble a mixed fleet of HVs, AVs, CVs, and CAVs in a connected traffic-control system. Field tests involving human drivers can also raise safety concerns, making these modelling studies particularly important; we want to identify potential problems now, and not when real lives are at stake.”
The paper, “Effects of Connectivity and Automation on Saturation Headway and Capacity at Signalized Intersections,” is published in Transportation Research Record.