What is the definition of speed vs density in traffic flow theory?

In traffic flow theory, speed vs density refers to the relationship between the speed of vehicles and the density of vehicles on a roadway. The correct definition states that as density decreases, speed tends to increase. This concept is grounded in the observation that low vehicle density allows for higher speeds because vehicles have more space to maneuver and are less constrained by the presence of other vehicles. Conversely, as density increases, the speed typically decreases due to the limitations imposed by the number of vehicles occupying the same space, leading to potential congestion.

This relationship is significant because it helps traffic engineers understand and predict traffic behavior, manage roadway capacity, and design better traffic systems to improve flow and reduce delays. Understanding this speed-density relationship is key in developing effective traffic management strategies and optimizing roadway designs to balance safety and efficiency.