Abstract
•There is a clear relationship between string stability and hysteresis.•String unstable platoons are hysteretic both for simulations and observed trajectories.•A new mechanism that causes hysteresis is investigated.•A new criterion for hysteresis partly explaining the connection to string stability.•The most string stable platoons produced poor flows during the acceleration phase.
The existence, source, and characteristics of hysteresis loops in traffic oscillations have been extensively investigated in the past decades. At the same time, the string stability of vehicle platoons has also attracted significant attention and has been considered the main cause of the spontaneously emerging oscillations. However, the relationship between these two seemingly related properties remains underexplored, leading to deficits in knowledge about the effect of microscopic controllers’ operations on macroscopic traffic characteristics. Filling this gap becomes important with the advent of driving automation and the possibility to make future road transport not only safer but also more efficient. This would however require setting proper requirements to their operations, able to ensure that they would generate an efficient traffic flow without limiting too much the freedom of the system developers.
By analyzing both simulation and real-world data, the present work has confirmed the existence of a strong correlation between hysteresis and string instability. In particular, string unstable platoons are found to be also hysteretic, while only a limited number of string stable platoons were hysteretic. Further, for a simple linear car following model, hysteresis loops are shown to emerge in the absence of inter-driver and/or intra-driver heterogeneity, which have been suggested as the main sources of hysteresis. This implies a new mechanism that can lead to hysteresis, emerging as a characteristic of the driving behavior. An analytical investigation of this mechanism shows a link to string instability, at least for the simple linear model used. Although further research will be needed in order to generalize the findings of this paper, the present results strongly suggest that introducing a requirement for string stability may lead to a significant reduction in the occurrence and the negative effects of traffic oscillations.