Requiem on the positive effects of commercial adaptive cruise control on motorway traffic and recommendations for future automated driving systems

Ciuffo, Biagio; Mattas, Konstantinos; Makridis, Michail; Albano, Giovanni; Anesiadou, Aikaterini; He, Yinglong; Josvai, Szilárd; Komnos, Dimitris; Pataki, Márton; Vass, Sándor; Szalay, Zsolt · 2021 · OpenAlex-citations

DOI: 10.1016/j.trc.2021.103305

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Summary

This study investigates the impact of commercially available Adaptive Cruise Control (ACC) systems on motorway traffic flow, energy consumption, and safety, using ACC as a proxy to anticipate potential issues with future Connected and Automated Vehicles (CAVs). While early simulations predicted that ACC would improve traffic stability and reduce congestion, the authors argue that real-world commercial implementations lack regulatory requirements for traffic flow optimization. To address the gap in experimental data, the European Commission’s Joint Research Centre conducted a comprehensive test campaign involving ten ACC-equipped vehicles from various manufacturers and powertrains (internal combustion, hybrid, and electric). The experiments were performed at the ZalaZONE proving ground in Hungary at low speeds (30–60 km/h), analyzing vehicle platoons under different ACC settings and vehicle orders. The methodology focused on four key metrics: ACC controller properties (time gap, response time, acceleration/deceleration rates), string stability, traffic hysteresis, and tractive energy consumption. String stability was assessed using $L_\infty$ norms to determine if speed perturbations amplified as they traveled upstream through the platoon. Traffic hysteresis, which indicates a loss in road capacity due to delayed flow recovery after deceleration, was quantified using trajectory data and macroscopic traffic variables. Energy consumption was evaluated by calculating tractive energy demand, comparing intra-platoon increases relative to the leading vehicle and inter-platoon differences between configurations with uniform short time gaps versus mixed time gaps. The results confirm that current commercial ACC systems are string unstable, meaning they amplify speed disturbances rather than dampening them. This instability leads to significant traffic hysteresis, reducing effective road capacity and introducing safety risks such as increased likelihood of rear-end collisions. Furthermore, the study found that ACC-driven vehicles consumed more tractive energy than human-driven counterparts or the leading vehicle in a platoon, primarily due to the erratic acceleration and deceleration patterns caused by string instability. The findings indicate that ACC systems, in their current form, do not deliver the promised benefits of smoother traffic or reduced energy use; instead, they exacerbate traffic oscillations and inefficiencies. The significance of this research lies in its implications for the development of future automated driving systems. The authors conclude that without specific functional requirements ensuring string stability and traffic flow compatibility, widespread adoption of ACC and similar Level 1–2 automation could degrade road safety and capacity. The paper advocates for introducing regulatory standards that mandate string stability for all automated systems entering the market. By learning from the limitations of current ACC implementations, policymakers and manufacturers can design CAVs that genuinely improve traffic dynamics rather than replicating the negative externalities observed in this study.

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