  We investigate dynamical properties of traffic flow using the stochastic
car-following model with modified optimal velocity on circular road. The safety
distance following the two-second rule and autonomous vehicles, acting as
agents, obeying simple requirements are incorporated into the model. The
dynamic safety distance increases in a light traffic condition where the
average driving velocity is high, while decreases in a dense traffic condition
in anticipation of slower traffic motion. The results show that the presence of
the agents can enhance overall velocity and traffic current of the system, and
postpone the traffic congestion. In a particular phase region, imposing a speed
limit enables the system to leave the congested flow phase. The
density-dependent speed limit in agent-free condition is obtained to achieve
the optimal traffic flow.
