In this study, the authors consider a family of six Automated Highway Systems (AHS) operational concepts. For each concept, the minimum inter- vehicle spacing that could be used for collision-free vehicle following, under different road conditions, is calculated. For architectures involving platoons, the authors also use the alternative constraint of bounded energy collisions to calculate the spacing that can be applied if collisions at a limited relative velocity were allowed. In every case, the minimum spacing in turn, is used to calculate the maximum possible capacity that could be achieved for each operational concept.