University of Minnesota Driven to Discover
U of MNUniversity of Minnesota
Center for Transportation Studies

A better cruise control

Technology is changing the way your cruise control works. By adding a radar range sensor and computer control, Adaptive Cruise Control (ACC) enables a car to automatically maintain a set distance from the vehicle ahead of it. Engineers hope that widespread adoption of ACC systems will eventually enable cars to cruise safely with minimal separation between them, resulting in higher road capacities without traffic disruptions.

However, questions remain about the stability of traffic flow when large numbers of ACC-equipped vehicles travel together under real-world highway conditions. A group of engineering researchers from the University of Minnesota explores this issue in a new report, published by the ITS Institute. Associate Professor Rajesh Rajamani of the Department of Mechanical Engineering, Assistant Professor David Levinson and Professor Panos Michalopoulos of the Department of Civil Engineering, and graduate students J. Wang, Kumaragovindhan Santhanakrishnan, and Xi Zou address the questions of whether the flow of vehicles using a constant-time-gap (CTG) spacing policy is stable under all conditions, including disruption by new vehicles entering the traffic stream, and whether a better spacing policy is feasible.

Under a constant-time-gap spacing policy, all vehicles using ACC maintain a fixed time interval between themselves and the next vehicle ahead of them; as vehicle speeds increase, the distance between vehicles also increases. Using computer simulations, some analyses of this policy have suggested that it can lead to traffic flow becoming unstable, meaning that disturbances in vehicle spacing (such as those caused by new vehicles entering a highway) can propagate upstream, potentially resulting in more serious disruptions.

The researchers examined mathematical models of CTG spacing effects and found that the stability of traffic flow is in fact dependent on boundary conditions—the initial configuration or “state” of vehicles on the highway when a simulation begins. In the real world, this means that certain configurations of vehicles will lead to the emergence of unstable traffic flow.

Having established the conditional instability of CTG spacing, the research team set out to explore the question of whether better spacing policies are possible. Their proposal, the variable-time-gap (VTG) policy, differs from the CTG policy in being a nonlinear, rather than linear, function of vehicle velocity. Models of traffic flow using this policy show that it is able to attenuate density disturbances caused by vehicles entering the highway, and that it leads to overall better traffic performance under many conditions. However, the researchers note that whatever spacing policy is chosen, instability will emerge when traffic density passes a critical limit.

More information on Automated Vehicle Control Algorithms and their Influence on Traffic is available.