  A universal relation has been established between the local energy transport
velocity along the direction of propagation and the group velocity of scalar
and vector-valued propagation-invariant spatiotemporally localized superluminal
and subluminal electromagnetic waves in free space. Under specific
restrictions, this relationship is very closely valid for physically realizable
almost propagation-invariant spatiotemporally confined subluminal and
superluminal electromagnetic fields. In both cases, although the group velocity
may be either superluminal or subluminal, the universal relation is in accord
with the well-established result that the upper limit of the energy transport
velocity is c, the speed of light in vacuum.
