The capacity and safety of freeways can potentially be increased substantially if the vehicles are operated in platoons, using automatic longitudinal control systems to maintain very small spacings (of the order of 1 meter) between vehicles. This paper explains many of the technical considerations in the design of such control systems, employing a general nonlinear simulation model to develop quantitative results. The effects on control system performance of external forces, process and measurement noise, and sampling and quantization of measurements are shown. The importance of acceleration and jerk limits is demonstrated, and examples are used to illustrate how the control system must be designed to accommodate variations in the severity of the maneuvers it is expected to execute, as well as variations in propulsion system dynamics.