Which model describes a linear relationship between speed and density under uninterrupted flow conditions?

Answer: B — Greenshield's Model

Greenshields' model describes a linear relationship between speed and density under uninterrupted flow. In this model, as density increases, speed falls linearly, with the speed at zero density equal to the free-flow speed and speed dropping to zero at a maximum jam density. Mathematically, v = vf (1 − k/kj), where vf is the free-flow speed and kj is the jam density. This straight-line behavior for speed versus density is what identifies the model. The other concepts don’t describe this linear speed–density link: saturation flow rate relates to the maximum vehicle passage rate at a signalized intersection, not a speed-density relationship; a phase is a term from traffic signal timing; and arterial highway refers to a road type rather than a speed-density model.