  In this paper, we characterize the effects of turning and merging maneuvers
of connected and/or automated vehicles (CAVs or AVs) on network traffic
instability using the macroscopic or network fundamental diagram (MFD or NFD).
We revisit the two-ring system from a theoretical perspective and develop an
integrated modeling framework consisting of different microscopic traffic
models of human-driven vehicles (HVs), AVs, and CAVs. Results suggest that
network traffic instability due to turning and merging maneuvers is an
intrinsic property of road networks. When the turning probability is low, CAVs
do not significantly change the NFD bifurcation, but scatter in both the
simulated link fundamental diagrams (FDs) and NFDs reduces leading to higher
and more stable network flows. When the turning probability is high,
non-cooperative AVs worsen network traffic instability - the NFD undergoes
bifurcation long before the critical density is reached. Results highlight the
important impact of cooperative merging on network traffic stability when AVs
are widely deployed in road networks.
