A Four-Factor Fundamental Diagram Model for Autonomous Vehicles: Flow, Speed, Density, and Energy

The large-scale integration of autonomous vehicles (AVs) has led to fundamental changes in the energy consumption evolution mechanism of new hybrid traffic flows, rendering conventional mapping between traffic flow parameters and energy invalid. This study develops a “flow–speed–density–energy” model for hybrid traffic composed of autonomous and human-driven vehicles (HVs). In this study, energy (kJ/km/h) is mathematically expressed as the product of the energy factor (kJ/km/veh) and traffic flow (veh/h). By constructing flow and energy factor curves with varying penetration rates and platoon intensities, the study derives four–factor diagrams for hybrid traffic flows. The analysis yields several key findings. First, for internal combustion engine vehicles (ICEVs), energy in HV platoons decreases with increasing speed, whereas in AV platoons it first decreases and then rises as flow intensifies under high-speed conditions. Second, energy is inversely correlated with density, reflecting the negative speed–density relationship. Under uncongested conditions, energy increases with flow, while in congested regimes, sharp reductions in the energy factor leads to overall energy decline. Finally, the primary difference between electric vehicles (EVs) and ICEVs lies in the low-speed range, where the energy of ICEVs decreases rapidly with increasing speed, whereas the energy of EVs increases due to the slower decline of the energy factor and faster flow growth. This study addresses the challenges in the operation and energy consumption of hybrid traffic flow at the macroscopic traffic level, providing foundational support for energy management.