Automated vehicle (AV) following control impacts traffic safety, mobility and stability. A recent empirical study [Shi and Li, 2020] showed that
 commercial AV following control became more unstable as the headway was set to a smaller value, implying a possible intrinsic trade-off between safety, mobility and stability aspects in AV following control design. This study aims to analytically explain the underlying vehicle control mechanism that dictates this trade-off. We investigate a parsimonious linear AV following model that captures the first-order parameters on the safety, mobility and stability aspects. The parsimonious model structure allows us to formulate and analytically solve a mathematical programming model on the worst-case safety buffer requirement for a given control time lag. The analytical solution clearly reveals the trade-off between the time lag and safety buffer that collectively determine the overall headway and thus the corresponding mobility performance such as road capacity.

 Further analysis shows how to balance this trade-off to theoretically minimize the overall AV following headway (or equivalently maximize road capacity) considering the needed safety buffer. Next, stability analysis reveals the trade-off between time lag and string stability of the study AV following model. The string stability results indicate that the minimum headway considering safety buffer alone may be hardly achieved due to stability concerns, and practical AV control needs to either allow a longer headway to trade for string-stable traffic or conversely accommodate moderate string-instability to trade for a smaller AV following headway. This finding also provides a possible explanation on observed string instability of commercial AV following control (e.g., adaptive cruise control) as a trade-off for a smaller headway. Overall, this study lays a new methodology foundation for incorporating safety in traffic flow analysis that traditionally focused on only mobility and stability. The findings yield a set of managerial insights into reasonable AV following design and its implications to emerging AV traffic management. Further, note that the proposed methodology is also applicable to the robust overshoot analysis for a general linear feedback control system. Thus, the findings and corresponding managerial insights are informative to robust design of feedback controls systems with collision safety concerns.      

Trade-off between safety, mobility and stability in automated vehicle following control: An analytical method

ASCE International Conference on Transportation & Development (ICTD 2021) is organized by the Transportation & Development Institute (T&DI) of ASCE to facilitate the exchange of information, knowledge, and best practices among transportation and development practitioners and researchers, public infrastructure owners, policymakers, government engineers and planners, operations managers, and leading academics from around the world.

Paper Proceedings: https://ascelibrary.org/doi/book/10.1061/ictd2021