Explain Greenshields' linear speed-density model.

Greenshields' model assumes linear relationship between speed and density: v = vf * (1 - k/kj), where vf = free-flow speed, kj = jam density, k = current density. From fundamental equation q = v*k: flow q = vf*k - (vf/kj)*k^2, a parabolic relationship with maximum flow (capacity) at k = kj/2 and v = vf/2. Model limitations: assumes uniform conditions and may not match field observations perfectly, but provides fundamental understanding of traffic behavior.