We study the stability and robustness of a large platoon of vehicles, where each vehicle is modeled as a double integrator, for two decentralized control architectures: predecessor following and symmetric bidirectional. In the predecessor-following architecture, the control action on each agent only depends on the information from its immediate front neighbor, whereas in the symmetric bidirectional architecture, it depends equally on the information from both its immediate front neighbor and back neighbor. We prove asymptotic stability of the formation for a class of nonlinear controllers with sector nonlinearity, with the linear controller as a special case. We show that the convergence rate of the predecessor-following architecture is much faster than that of the symmetric bidirectional architecture. However, the predecessor-following architecture suffers high algebraic growth of initial errors. We also establish scaling laws (with N ) of certain H 1 norms of the formation that measure its robustness to external disturbances for the linear case. It is shown that the robustness performance grows geometrically in N for predecessor-following architecture but only polynomially in N for symmetric-bidirectional architecture. Extensive numerical simulations are conducted to verify the predictions for the linear case and empirically estimate the corresponding performance metrics for a saturation-type nonlinear controller. On the basis of the analytical and numerical results, it is seen that the symmetric bidirectional architecture outperforms the predecessor-following architecture in all measures of performance. Within the predecessor-following architecture, the nonlinear controller is seen to perform better in general than the linear one. A number of design guidelines are provided on the basis of these conclusions. motivations for automated platooning is to achieve higher highway capacity by making cars move with a small inter-vehicle separation, there is a need to study the constant spacing policy. It was shown in [11,14,15] that, with constant spacing policy, the leader's information need to be broadcasted to the following vehicles to assure string stability. Nevertheless, the inevitable time delay and package drop in broadcasting the leader's information will cause string instability [16]. This leads to the study of decentralized control architecture, that is, each vehicle can only use measurements of relative position and/or velocity with respect to its nearest neighbors.Two decentralized control architectures that are commonly examined are predecessor-following and bidirectional architectures. In the predecessor-following architecture, the control action on each vehicle only depends on the relative information from its immediate predecessor, that is, the vehicle in front of it. In the bidirectional architecture, the control depends on the relative information from both its immediate predecessor and follower. Within the bidirectional architecture, the most commonly analyzed case is the symmetric bidirectional arch...