The units of transportation measurement describes the unit of measurement used to measure the quantity and traffic of transportation used in transportation statistics, planning, and their related fields.
The currently popular units are:
Passenger-distance is the distance (km or miles) travelled by passengers on transit vehicles; determined by multiplying the number of unlinked passenger trips by the average length of their trips.
A system may carry a high number of passengers per distance (km or mile) but a relatively low number of passengers per bus hour if vehicles operate in congested areas and thus travel at slower speed.
A transit system serving a community with a widely dispersed population must operate circuitous routes that tend to carry fewer passengers per distance (km or mile). A higher number is more favorable.
A simple unit of freight is the kilogram-kilometre (kgkm), the service of moving one kilogram of payload a distance of one kilometre.
Outside the USA and internationally the metric units (pkm and tkm) are used. (In aviation where United States customary units are still widely used, the International Air Transport Association (IATA) releases its statistics in the metric units.)
In the USA, sometimes United States customary units are used.
(Please comment on the usage in the United Kingdom.)
The dimension of the measure is the product of the payload mass and the distance transported.
A semi truck traveling from Los Angeles to Chicago (approximate distance 2,015 miles) carrying 14 short tons of cargo delivers a service of 14 * 2,015 = 28,210 ton-miles of freight (equal to about 41,187 tkm).
Intermodal container traffic is commonly measured in twenty-foot equivalent units (TEUs), rather than cargo weight, e.g. a TEU-km would be the equivalent of one twenty-foot container transported one kilometer.
Transportation density can be defined as the payload per period, say passenger / day or tonne / day. This can be used as the measure of intensity of the transportation on a particular section or point of transportation infrastructure, say road or railway. This can be used in comparison with the construction, running costs of the infrastructure.
Here is an example of transportation statistics.