Are commercially implemented adaptive cruise control systems string stable? (1905.02108v1)

Abstract: In this article, we assess the string stability of seven 2018 model year adaptive cruise control (ACC) equipped vehicles that are widely available in the US market. Seven distinct vehicle models from two different vehicle makes are analyzed using data collected from more than 1,200 miles of driving in car-following experiments with ACC engaged by the follower vehicle. The resulting dataset is used to identify the parameters of a linear second order delay differential equation model that approximates the behavior of the black box ACC systems. The string stability of the data-fitted model associated with each vehicle is assessed, and the main finding is that all seven vehicle models have string unstable ACC systems. For one commonly available vehicle model that offers ACC as a standard feature on all trim levels, we validate the string stability finding with a multi-vehicle platoon experiment in which all vehicles are the same year, make, and model. In this test, an initial disturbance of 6 mph is amplified to a 25 mph disturbance, at which point the last vehicle in the platoon is observed to disengage the ACC. The data collected in the driving experiments is made available, representing the largest publicly available comparative driving dataset on ACC equipped vehicles.

• The paper shows that all seven commercial ACC models amplify disturbances, indicating inherent string instability.
• It employs 1,200 miles of controlled car-following tests and delay differential equations to model vehicle dynamics.
• The results highlight a need to redesign ACC control laws, with time-headway and connectivity strategies as promising solutions.

Evaluating the String Stability of Commercial Adaptive Cruise Control Systems

The paper "Are commercially implemented adaptive cruise control systems string stable?" presents a comprehensive paper on the string stability of seven commercially available 2018 model year adaptive cruise control (ACC) vehicles. The research answers a crucial question in vehicular automation: do current commercial ACC systems inherently enhance or destabilize traffic flow when subjected to small disturbances?

Overview of Research Methodology

The paper methodically tests seven distinct vehicle models from two major manufacturers. Data were collected over 1,200 miles of driving under controlled car-following scenarios with ACC engaged. Each vehicle's dynamics were modeled using a linear second order delay differential equation (DDE) approach. The paper explores the theoretical foundations of string stability, characterizing whether disturbances in a line of vehicles are attenuated or exacerbated as they progress backward in the queue.

The research extends the analysis to a practical setting with an experimental platoon test. One vehicle make, for which ACC is standard across all models, undergoes a rigorous multi-vehicle platoon setup wherein the impact of disturbances is monitored across the fleet. A 6 mph speed disturbance results in a magnified disturbance of 25 mph at the rear vehicle, illustrating the string instability in practice.

Key Findings

The uniformity in findings across different models is striking. All seven vehicles exhibited string instability—a condition where disturbances are amplified rather than dissipated as they propagate through the vehicle line. Such a characteristic is problematic for traffic flow stability, potentially contributing to the well-documented phenomenon of phantom traffic jams, mirroring issues seen with human-driven vehicles.

Implications and Future Directions

The immediate implication is evident: current ACC implementations on commercial vehicles fail to mitigate traffic oscillations, challenging any assumptions of inherent benefits in traffic flow stability from these technologies. This research supports a notion that merely introducing automated systems like ACC does not ensure enhanced traffic efficiency or stability unless such systems are consciously engineered for string stability.

From a theoretical standpoint, this work reiterates the importance of carefully tuned control laws that account for system stability. While constant-spacing control policies have been critiqued for encountering string instability issues, time-headway control strategies appear more promising, yet remain under-explored in commercial implementations.

Future research could look into enhancing system connectivity—an avenue where connected vehicle technology could allow early detection and coordinated response to perturbations. Furthermore, exploring hybrid control policies that integrate both time headway and connectivity features could provide valuable insights.

Conclusion

The exhaustive analysis presented underscores critical concerns regarding the readiness of commercially available ACC systems to contribute to improving road traffic conditions. The work pushes boundaries by including a comparison via large-scale experimental setups and sophisticated mathematical modeling, crucial for paving the way toward more effective deployment of vehicular automation technologies. As ACC systems evolve, string stability should remain a key focus area to ensure safety, efficiency, and sustainability in future transportation networks.