**Speed** of a motion is the time rate of length change, that is, the length interval with respect to a timeline interval without regard to direction. **Pace** of a motion is the space rate of time change, that is, the time interval with respect to a baseline interval without regard to direction.

The symbol for speed is *v* = Δ*s*/Δ*t* and for pace is *u* = Δ*t*/Δ*s*. Instantaneous speed is d*s*/d*t*. Puncstancial pace is d*t*/d*s*.

There are two kinds of mean speed or pace: the *time mean* and the *space mean*. The time mean is the arithmetic mean if the denominators are a common time interval. The space mean is the arithmetic mean if the denominators are a common space interval. The time mean is the harmonic mean if the denominators are a common space interval. If the denominators are a common time interval, the space mean is the harmonic mean.

The *time mean speed* (TMS) is the arithmetic mean of speeds with a common time interval. The *time mean pace* (TMP) is the harmonic mean of paces with a common time interval. For example, the travel distance for vehicles on a highway during a time period is measured. The time mean speed or pace may then be calculated.

The *space mean pace* (SMP) is the arithmetic mean of paces with a common space interval. The *space mean speed* (SMS) is the harmonic mean of speeds with a common space interval. For example, the travel time for vehicles over a length of highway is measured. The space mean speed or pace may then be calculated.

If the speed of light is measured by reflection between two points, the result is a time mean speed of the transmission and reflection speeds. For several of these measurements the result is a space mean of time mean speeds. The second postulate of relativity says that all of these speeds should be equal.

The travel distance for vehicles over a time interval may be measured by probe vehicles that record odometer readings for two points in time. The travel time for vehicles over a length of highway may be measured by a time stamp for each vehicle at two points on the highway. This may be done with sensors placed a short distance apart so the consecutive time stamps are for the same vehicle.

Spot speeds are instantaneous speeds of vehicles at a point on the roadway. They may be measured by a radar device on the roadside. The mean speed for spot speeds is a time mean speed.

In symbols:

If a beam of light is reflected back from a known distance, the *two-way space mean speed of light* may be calculated. From measurements such as this the *one-way speed of light* in a vacuum has been defined as a constant equal to 299,792,458 m/s.