Travel, that is, the movement of something, includes transporting and signalling. To transport means to take something (e.g., people or goods) from one place to another by means of a vehicle or the like (e.g., a car). To signal means to transmit information or instructions from one place to another through a medium or the like (e.g., sound).
A mode of travel is a means, technology, or technique for moving something. Travel modes may be distinguished by whether they are on or through a solid (e.g., land), on or through a liquid (e.g., water), through a gas (e.g., air), or in a vacuum (e.g., outer space).
A mode of travel has a free-flow speed, which is the speed attained in which there are no impediments to travel in that mode. This is the highest normal speed in the mode but may not be the highest possible speed. If local conditions (e.g., topography) do not exert significant influence, the free-flow speed serves as a reference speed for the mode because it is homogeneous and isotropic.
There are two basic perspectives on travel and the measurement of travel: (1) the most common perspective looks from a state of rest and observes something traveling relative to it; (2) the second perspective looks from a state of travel and observes something that is not moving (e.g., a landscape). The basic measure for perspective (1) is velocity, the change in displacement traveled per unit of travel time. The basic measure for perspective (2) is inverse velocity, the change in travel time per unit of distance traveled.
The first perspective (1) is the spatial perspective because it is from a state of rest, which is associated with space, with something that seems to be there apart from time. The second perspective (2) is the temporal perspective because it is from a state of travel, which is associated with the passage of time, with something that takes time. The Galilean and Lorentz transformations apply to the spatial perspective but as we have seen there are similar transformations that apply to the temporal perspective.