Speed (‘v’) is the distance travelled by a vehicle per unit time and is typically expressed as either metres per second (m/s) or kilometres per hour (km/h).
The average speed of a stream of vehicles may be expressed as either the time mean speed or the space mean speed, which are defined below.
Time mean speed, vt, is the arithmetic mean of the measured speeds of all vehicles passing a given point during a given time interval. Such individual measured speeds are called ‘spot speeds’.
Space mean speed, vs, is the arithmetic mean of the measured speeds of all vehicles within a given length of lane or carriageway, at a given instant of time.
In Section 2.3, it is shown that, for any given traffic stream, space mean speed can be estimated as the harmonic mean (the inverse of the mean of speed inverses as in Equation 2.5) of the same spot speeds whose arithmetic mean is the time mean speed.