Spacing and capacity evaluations for different AHS concepts

Abstract

In Automated Highway Systems (AHS), vehicles will be able to follow each other automatically by using their own sensing and control systems, effectively reducing the role of the human driver in the operation of the vehicle. Such systems are therefore capable of reducing one source of error, human error, that diminishes the potential capacity of the highways and in the worst case becomes the cause of accidents. The inter-vehicle separation during vehicle following is one of the most critical parameters of the AHS system, as it affects both safety and highway capacity. To achieve the goal of improved highway capacity, the inter-vehicle separation should be as small as possible. On the other hand, to achieve the goal of improved safety and elimination of rear end collisions, the inter-vehicle separation should be large enough that even under, a worst case stopping scenario, no vehicle collisions will take place. These two requirements demand diametrically opposing solutions and they have to be traded off. Since safety cannot be compromised for the sake of capacity, it becomes a serious constraint in most AHS design decisions. The trade-off between capacity and safety gives rise to a variety of different AHS concepts and architectures. In this study we consider a family of AHS operational concepts. For each concept we calculate the minimum inter-vehicle spacing that could be used for collision-free vehicle following, under different road conditions. For architectures involving platoons we also use the alternative constraint of bounded energy collisions to calculate the minimum spacing that can be applied if we allowed collisions at a limited relative velocity in case of emergency stopping. The minimum spacing is used to calculate the maximum possible capacity that could be achieved for each operational concept.