English: Fundamental hypothesis of Kerner’ three-phase traffic theory: (a) Qualitative representation of free flow states (F) and 2D-region of steady states of synchronized flow (dashed region S) on a multi-lane road in the flow-density plane. (b) A part of the 2D-region of steady states of synchronized flow shown in (a) in the space-gap-speed plane (dashed region S). In (b), ${\displaystyle G}$ and ${\displaystyle g_{\rm {safe}}}$, are respectively a synchronization space gap and safe space gap between two vehicles following each other.