Drone Delivery: Nature of Traffic Conflicts in Constrained Urban Airspace Environments

Drone-based delivery is likely to reduce energy and greenhouse gas emissions associated with the transport of small express packages, fast-food meals and medicines, especially when deployed in large-scale in urban areas. However, it is an enormous challenge to facilitate such high traffic densities in constrained urban environments. A recent study applied the principles of traffic alignment and segmentation to the constrained urban airspace of Manhattan, New York, in an effort to mitigate conflict probability. In that study, two novel airspace concepts were proposed, namely, the two-way and one-way concept. Both concepts employed a heading-altitude rule to vertically segmented traffic with respect to their travel directions. In addition, the one-way concept also featured horizontal constraints to promote unidirectional traffic flow. These concepts bear resemblance to that of road-based traffic. Further, both concepts featured transition altitudes to accommodate turning flights that need to decelerate to safely perform turns at intersections. The comparative study showed the one-way airspace configuration to be more effective than the two-way concept in terms of safety. In the pursuit of demonstrating our understanding of the intricate differences between the two-way and one-way airspace configurations, this paper aims to explore and analyse salient conflict properties. By using fast-time simulation experiments, the different types of conflicts are captured and analysed for multiple traffic demand levels. Our results suggest that conflicts are largely caused by flights climbing or descending to their respective altitude layers. For both concepts, the merging conflicts consisted of in-trail and crossing conflicts, while the two-way also contained a high proportion of head-on conflicts. Our study therefore sheds light on the different categories of conflicts that could help guide future research in airspace design in constrained urban areas.