Spacing and Capacity Evaluations for Different AHS Concepts
Alexander Kanaris, Petros Ioannou, Fu-Sheng Ho
Automated Highway Systems
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 collisionfree 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. 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 six 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 spacing that can be applied if we allowed collisions at a limited relative velocity. In every case, the minimum spacing in turn, is used to calculate the maximum possible capacity that could be achieved for each operational concept. Executive Summary In this paper we analyze and evaluate the braking performance of different vehicle classes under six different Automated Highway Systems (AHS) operational concepts. For each operational concept we calculate the minimum inter-vehicle spacing that could be applied in order to achieve collision-free vehicle following under different road conditions such as wet and dry road surfaces. In addition to collision-free environments, for AHS architectures involving platoons, we also apply the alternative constraint of bounded energy collisions to calculate the spacing that can be applied if we wanted to allow collisions at a specific limit of relative velocity. In every case, the minimum spacing is used to calculate the maximum achievable capacity for each operational concept, thus opening the way for safety, risk, cost and performance tradeoff analysis of different AHS operational concepts. The tools that were developed during this study allow users to parameterizc and customize the vehicle braking scenario that will be considered as the worst case braking scenario because, obviously, different braking scenarios imply different spacing requirements and different capacity levels. To support our choice of parameters for the worst case .braking scenario we have applied in each case, we considered and included in this paper tables of vehicle braking performance data derived from road tests performed by MHTSA and by the leading consumer magazines. Almost equal in importance to the deceleration performance potential of the vehicles involved is the timing of the braking command, which involves detection, communication and actuation delays. These delays vary depending on the AHS operational concept that will be chosen and the components that will be employed. Our choice of timing parameters was based on sensor-actuatorcommunication technology limitations and is supported by vehicle tests performed by the authors and by other researchers in the PATH program. While the numerical results we obtained apply to nothing but the specific examples that we studied and the parameter choices we made, the methodology and tools we developed can easily be applied in order to evaluate the performance and limitations of any variant of these examples. Furthermore, by meticulously maintaining a level of consistency in the choice of parameters we made, we have obtained results that can be useful in ranking the relative merits of the different candidate AHS operational concepts.