Density-- the number of vehicles occupying a road lane per unit length at a given instant.
Flow-- the number of vehicles passing a point per unit of time; often called volume when the time unit is one hour.
Gap-- the time interval between the passage of consecutive vehicles moving in the same stream, measured between the rear of the lead vehicle and the front of the following vehicle.
Headway-- the time interval between passage of consecutive vehicles moving in the same stream, measured between corresponding points (e.g. front bumper) on successive vehicles.
Interrupted Flow-- occurs when flow is periodically interrupted by external fixtures, primarily traffic control devices.
Jam Density-- the density when speed and flow are zero.
PHF (Peak Hour Factor)-- This describes the relationship between hourly volume and the maximum rate of flow within the hour: PHF = hourly volume/maximum rate of flow. For the 15 minute periods, PHF = volume/4 x (maximum 15 minute volume within the hour)
Shockwaves-- Shockwaves occur as a result of differences in flow and density which occur when there are constrictions in traffic flow. These constrictions are called bottlenecks. The speed of growth of the ensuing queue is the shockwave, and is the difference in flow divided by the difference in density.
Space Mean Speed-- the arithmetic mean of the speed of those vehicles occupying a given length of road at a given instant.
Spacing-- the distance between vehicles moving in the same lane, measured between corresponding points (front to front) of consecutive vehicles.
Speed-- the time rate of change of distance.
Time Mean Speed-- the arithmetic mean of the speed of vehicles passing a point during a given time interval.
Travel Time-- the total time required for a vehicle to travel from one point to another over a specified route under prevailing conditions.
Uninterrupted Flow-- occurs when vehicles traversing a length of roadway are not required to stop by any cause external to the traffic stream, such as traffic control devices.
Volume-- Traffic volume is the most basic and widely used parameter in traffic engineering, vehicles per mile, or vehicles per kilometer.