  Vehicle platooning has the potential to significantly improve traffic
throughput and reduce fuel consumption and emissions and thus has attracted
extensive attention recently. In this study, we propose a hybrid vehicle
platoon system for the connected and automated vehicles (CAVs). First, a hybrid
spacing policy combining the constant time gap (CTG) and constant spacing (CS)
is formulated for the proposed platoon system, where the leader adopts the CTG
and the followers use the CS policy. Based on the h2-norm string stability
criteria, the notions of exogenous-head-to-tail string stability and hybrid
string stability are newly introduced, and the sufficient conditions of the
hybrid string stability in the frequency domain are derived using the Laplace
transform. Numerical experiments are conducted to validate the hybrid string
stability. Moreover, two typical scenarios and several measurements of
effectiveness (MOE) are adopted to verify the effectiveness of the proposed
hybrid platoon system in various aspects. The results show that the hybrid
platoon system performs better than the CS-based platoon system. It also
indicates that the hybrid platoon system has obvious advantages over the
CTG-based platoon system under the periodical fluctuation scenario and it is
also comparable to the CTG-based platoon system under the large deceleration
and acceleration scenario. The findings have demonstrated the merits of the
combined implementation of CTG and CS policy in enhancing the performance and
applicability of the platoon system for CAVs.
