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

As a freeway-driving strategy, jam-absorption driving (JAD) clears a shock wave (i.e., an upstream-moving congestion wave) by modulating the velocity of a single vehicle (called the absorbing vehicle) upstream of the wave, and improves traffic performances such as fuel consumption and collision risk. However, the deceleration of the absorbing vehicle causes trailing vehicles to slow down, generating secondary shock waves that undermine these benefits. 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, which are 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 using a traffic stream composed of 2000 vehicles, including five SVs, revealed that SD damped perturbations caused by the absorbing vehicle and prevented secondary shock waves, consistent with the head-to-tail string stability criterion. The combination of JAD and SD reduced total fuel consumption by 109 kg and collision risk (inverse time-to-collision) by 1940 compared with the JAD-only method, but increased total travel time by 49.3 hours. Reverting the extended time gap to its initial value reduced total travel time by 27.1 hours while maintaining low collision risk compared with the non-reverting method, albeit with increased total fuel consumption by 113 kg. SD alone could not eliminate the target shock wave. The results indicate that combining JAD and SD effectively eliminates the target shock wave while suppressing the secondary shock waves with guaranteed string stability even with only six CAVs (an absorbing vehicle and five SVs) out of 2000 vehicles, which is consistent with low CAV penetration rates anticipated in early implementation stages.