A field experiment in Yokohama, Japan, reveals the existence of a macroscopic fundamental diagram (MFD) linking space-mean flow, density, and speed on a large urban scale. The study combined data from fixed detectors and GPS-equipped taxis to analyze traffic patterns across the entire network. While fixed detectors showed scattered data, aggregating taxi data revealed a smoothly declining curve, suggesting the presence of an MFD. The analysis showed that space-mean speeds and densities closely followed this curve, with deviations attributed to experimental error. The study also found a fixed relationship between space-mean flows and trip completion rates, which dynamically measure accessibility. The MFD was validated using taxi data, which showed that the relationship between production (flow × network length) and accumulation (density × network length) holds across different demand conditions. The study demonstrated that the trip completion rate is proportional to production, and that perimeter control strategies can improve accessibility. The results suggest that large urban networks behave predictably and independently of their origin-destination tables, provided accumulation is monitored. The findings support the use of MFDs for traffic management, including pricing, rationing, and perimeter control. The study highlights the importance of continuous traffic monitoring and the potential of GPS-equipped vehicles as probes for traffic state estimation. Future research should focus on refining control strategies and understanding the impact of infrastructure on MFDs. The results indicate that MFDs can be used to improve accessibility and manage traffic demand effectively.A field experiment in Yokohama, Japan, reveals the existence of a macroscopic fundamental diagram (MFD) linking space-mean flow, density, and speed on a large urban scale. The study combined data from fixed detectors and GPS-equipped taxis to analyze traffic patterns across the entire network. While fixed detectors showed scattered data, aggregating taxi data revealed a smoothly declining curve, suggesting the presence of an MFD. The analysis showed that space-mean speeds and densities closely followed this curve, with deviations attributed to experimental error. The study also found a fixed relationship between space-mean flows and trip completion rates, which dynamically measure accessibility. The MFD was validated using taxi data, which showed that the relationship between production (flow × network length) and accumulation (density × network length) holds across different demand conditions. The study demonstrated that the trip completion rate is proportional to production, and that perimeter control strategies can improve accessibility. The results suggest that large urban networks behave predictably and independently of their origin-destination tables, provided accumulation is monitored. The findings support the use of MFDs for traffic management, including pricing, rationing, and perimeter control. The study highlights the importance of continuous traffic monitoring and the potential of GPS-equipped vehicles as probes for traffic state estimation. Future research should focus on refining control strategies and understanding the impact of infrastructure on MFDs. The results indicate that MFDs can be used to improve accessibility and manage traffic demand effectively.