What is the fundamental relationship between traffic flow, speed, and density?

The fundamental traffic flow equation is: Flow (q) = Speed (v) x Density (k). Flow is vehicles per hour, speed is average velocity, and density is vehicles per distance. At zero density, speed is free-flow but flow is zero. As density increases, flow increases until capacity (maximum flow) is reached. Beyond this, congestion causes both speed and flow to decrease. This relationship forms the basis for traffic analysis and capacity determination.