Small unmanned aerial vehicles play increasingly important roles in many industries and organizations in the United States and around the globe. As usage becomes more widespread, airspace will become increasingly crowded. These high-density unmanned aerial vehicle (UAV) situations will pose many unique challenges, including in conflict resolution. Using agent-based models, conflict resolution and the well-clear and action-distance concepts for small multirotor UAVs in dense environments were explored. Specifically, the safety and efficiency of several collision-avoidance mechanisms under different possible densities, accelerations, and action distances were investigated. The results suggest that selecting appropriate action distances and collision-avoidance mechanisms will be vital to maintaining safety and efficiency. Interestingly, excessive action distances quickly degrade performance: using a simple method under high-density conditions (), well-clear distances above approximately 750 m have extremely low efficiency ( as many deliveries accomplished in 5000 simulation hours compared to lower distances). More advanced algorithms also suffer from low efficiency at higher () response distances. Different definitions for near-midair collisions and their effects on system safety and well-clear distance are discussed.