  Platooning of multiple autonomous vehicles has attracted significant
attention in both academia and industry. Despite its great potential,
platooning is not the only choice for the formation of autonomous vehicles in
mixed traffic flow, where autonomous vehicles and human-driven vehicles (HDVs)
coexist. In this paper, we investigate the optimal formation of autonomous
vehicles that can achieve an optimal system-wide performance in mixed traffic
flow. Specifically, we consider the optimal $\mathcal{H}_2$ performance of the
entire traffic flow, reflecting the potential of autonomous vehicles in
mitigating traffic perturbations. Then, we formulate the optimal formation
problem as a set function optimization problem. Numerical results reveal two
predominant optimal formations: uniform distribution and platoon formation,
depending on traffic parameters. In addition, we show that 1) the prevailing
platoon formation is not always the optimal choice; 2) platoon formation might
be the worst choice when HDVs have a poor string stability behavior. These
results suggest more opportunities for the formation of autonomous vehicles,
beyond platooning, in mixed traffic flow.
