We investigate the effect of a number of design alternatives on the safety and capacity of an Automated Highway System. Our methodology makes use of two computational tools, designed to highlight the fundamental limitations of the vehicle dynamics, sensing and control strategies and inter-vehicle communication. The first tool produces the minimum spacing necessary for two vehicles not to collide, as a function of their state and capabilities. The second tool investigates the multiple collisions that may occur in a string of vehicles if the spacing requirements of the first tool are violated. The example we have in mind is the emergency deceleration of a platoon of vehicles. We use the tools to establish limits on the safety and throughput that can be expected if different automated highway concepts are implemented. Our results indicate that on an Automated Highway System that supports platooning, severe intra-platoon collisions may occur under emergency situations. We analyze the effect of the collisions on both safety and throughput and investigate the sensitivity of these quantities to a number of design parameters. We identify as the most important parameters inter-vehicle coordination, on-line braking capability estimation and platoon organization in terms of braking capability.