What formula calculates the number of vehicles per mile per lane?

The correct choice, which defines density in terms of volume and speed, is based on the fundamental relationship in traffic flow theory. Traffic density is expressed as the number of vehicles per mile per lane, which aligns perfectly with the formula provided.

In this context, density (D) is defined as the number of vehicles (V) occupying a given length of road (L) over the unit time, often represented as vehicles per mile. When relating density to flow and speed, this formula effectively captures the dynamic relationships between these traffic parameters.

When traffic persists at a constant speed, the flow (or volume) can be conceptualized using the equations of traffic flow theory, whereby flow is the product of density and speed. Thus, speed and volume together contribute to understanding how many vehicles are likely to occupy a given road segment at any point in time.

In summary, the correct choice encapsulates how density is calculated and provides a crucial foundation for analyzing traffic behavior, making it fundamental to transportation engineering and planning considerations.