Assessing Strong String Stability of Constant Spacing Policy under Speed Limit Fluctuations
It identifies a fundamental limitation in platoon control design for automated vehicles under varying speed limits, relevant to traffic stability and efficiency.
The paper proves that for ACC-based platoons with constant spacing policy, no controller can simultaneously maintain constant spacing and track a speed limit profile that varies with location, due to inherent stability constraints.
The speed limit changes frequently throughout the transportation network, due to either safety (e.g., change in geometry) or congestion management (e.g., speed harmonization systems). Any abrupt reduction in the speed limit can create a shockwave that propagates upstream in traffic. Dealing with such an abrupt reduction in speed limit is particularly important while designing control laws for a platoon of automated vehicles from both stability and efficiency perspectives. This paper focuses on Adaptive Cruise Control (ACC) based platooning under a constant spacing policy, and investigates the possibility of designing a controller that ensures stability, while tracking a given target velocity profile that changes as a function of location. An ideal controller should maintain a constant spacing between successive vehicles, while tracking the desired velocity profile. The analytical investigations of this paper suggest that such a controller does not exist.