This paper addresses the problem of longitudinal control of vehicular strings, otherwise known as vehicular platoons. We set up the control problem as an H2 robust control problem, where the objective is to allow vehicles to travel in close ranks safely and comfortably, to respond to reference commands, and to operate well under uncertain road and wind conditions. We address the issue of centralized versus decentralized control strategies. We show that vehicular strings are a plant with Toeplitz like structure, optimal centralized H2 controllers inherit that structure. The implications of this structre to the relative performance of diierent control strategies is analyzed. We propose a certain \almost" decentralized control strategy that has near centralized optimal performance. The results are simulated and illustrated on vehicular strings of varying sizes from 10 to 50 vehicles.