Trade-off between safety, mobility and stability of automated vehicles

Automated vehicle (AV) following control impacts traffic safety, mobility and stability. Our empirical study 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. We further analytically explained the underlying vehicle control mechanism that dictates this trade-off. We investigated a parsimonious linear AV following model that captures the first-order parameters on the safety, mobility and stability aspects. The string stability results indicate that the minimum headway considering the 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. These findings are verified by field experiments with production vehicles. Vehicles of different powertrains (e.g., electric vehicles vs. internal combustion engines) are compared. Methods of improving vehicle performance are explored. This work also provides a possible explanation of observed string instability of commercial AV following control (e.g., adaptive cruise control) and provides thoughts on improving existing technology and transportation systems.