What does Greenshield's Model express?

Greenshield's Model is a fundamental concept in traffic flow theory that illustrates the relationship between traffic speed and vehicle density. It posits that as vehicle density increases, the speed of the vehicles typically decreases in a linear manner. This linear relationship indicates that there is an optimal density at which speed is maximized, and beyond that point, congestion starts to set in, leading to reduced speed.

The model is instrumental in understanding how traffic behaves under different conditions, helping traffic engineers and planners optimize roadway designs and manage traffic flow efficiently. This is particularly important when analyzing how to mitigate congestion and improve overall travel times in urban areas.

Other options, while related to traffic and transportation, do not accurately describe the main focus of Greenshield's Model. For instance, the relationship between volume and speed is a more complex interaction that is not strictly linear, and traffic accidents related to density relate to safety considerations rather than direct speed-density relationships. Finally, delays at intersections are often calculated using different methodologies and do not reflect the fundamental linear relationship Greenshield's Model describes.