Which metric represents the number of vehicles per hour that could cross the line if the signal remained green all of the time?

• A. Phase
• B. Demand
• C. Arterial Highway
• D. Saturation Flow Rate

Answer: (D)

The key idea is saturation flow rate—the maximum number of vehicles that can pass a signalized line per hour when the green phase could continue indefinitely, with no red time or yellow clearance losses. It represents the theoretical throughput of an approach under ideal green conditions, determined by driver behavior, vehicle spacing, and acceleration, and it’s expressed per lane (vphpl). Multiply by the number of lanes to get the total flow for the approach. This is different from demand, which is how many vehicles actually want to pass, or from a signal phase, which is just a stage in the cycle. It’s also not a road type. So the metric described is saturation flow rate.