SpeedFlowDensity Relationship
Speed, flow, and density are all related to each other. The relationships between speed
and density are not difficult to observe in the real world, while the effects of speed and
density on flow are not quite as apparent.
Under uninterrupted flow conditions, speed, density, and flow are all related by the
following equation:
q = k*v
Where
q = Flow (vehicles/hour)
v = Speed (miles/hour, kilometers/hour)
k = Density (vehicles/mile, vehicles/kilometer)
Because flow is the product of speed and density, the flow is equal to zero when one or
both of these terms is zero. It is also possible to deduce that the flow is maximized at
some critical combination of speed and density.
Two common traffic conditions illustrate these points. The first is the modern traffic
jam, where traffic densities are very high and speeds are very low. This combination
produces a very low flow. The second condition occurs when traffic densities are very low
and drivers can obtain free flow speed without any undue stress caused by other vehicles
on the roadway. The extremely low density compensates for the high speeds, and the
resulting flow is very low.
