Time headway and spacing are referred to as microscopic measures because they describe characteristics specific to individual pairs of vehicles within the traffic stream. Measures that describe the traffic stream as a whole, such as flow, average speed, and density, are referred to as macroscopic measures. As indicated by the preceding equations, the microscopic measures can be aggregated and related to the macroscopic measures.
Based on the definitions presented, a simple identity provides the basic relationship among traffic flow, speed (space-mean), and density (denoting space-mean speed, su as simply u for notational convenience):
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q uk= (5.14)
q = flow, typically in units of veh/h, u = speed (space-mean speed), typically in units of mi/h, and k = density, typically in units of veh/mi.