Suppressing secondary shock waves in jam-absorption driving via string-stable support vehicles

As a freeway-driving strategy, jam-absorption driving (JAD) clears a traffic shock wave (stop-and-go wave) by slowing down a single vehicle, called the absorbing vehicle. However, JAD may destabilize the traffic flow upstream of this vehicle, generating secondary shock waves. This study proposes a method to suppress secondary shock waves by controlling the behavior of connected and automated vehicles (CAVs) upstream of the absorbing vehicle, called support vehicles (SVs). A string-stability-based control method is applied in which SVs dynamically extend their time gaps to provide support driving (SD) for JAD. Numerical simulations revealed that SD damped perturbations caused by the absorbing vehicle and prevented secondary shock waves, consistent with the headto- tail string stability criterion. Combining JAD and SD reduced fuel consumption and collision risk compared with the JAD-only method, but increased travel time, across a wide range of key parameters. Reverting the extended time gap to its initial value reduced travel time while maintaining low collision risk compared with the non-reverting method, albeit with increased fuel consumption. Besides, our method is compared with consecutive JAD, and its practicality is discussed. Thus, combining JAD and SD effectively eliminates the target shock wave while suppressing secondary shock waves with guaranteed string stability.