Units of transportation measurement
Units of Transportation Quantity 
The currently popular units are:
Length of Journey 
- kilometre (km) or kilometer is a Metric unit used to measure the length of a journey outside the USA;
- the international statute mile (mi) is used in the USA; 1 mi = 1.609344 km
- nautical mile is rarely used to derive units of transportation quantity.
Traffic flow 
- vehicle-kilometer as a measure of traffic flow, determined by multiplying the number of vehicles on a given road or traffic network by the average length of their trips measured in kilometers.
- vehicle-mile same as before but measures the trip expressed in miles.
Payload Quantity 
- passenger or person (p)
Passenger-distance is the distance (km or miles) traveled by passengers on transit vehicles; determined by multiplying the number of unlinked passenger trips by the average length of their trips.
- passenger-kilometre or pkm internationally;
- passenger-mile (or pmi ?) sometimes in the USA; 1 pmi = 1.609344 pkm
Passengers per bus hour 
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.
Passengers per bus distance 
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-kilometer (kgkm), the service of moving one kilogram of payload a distance of one kilometer.
Payload Quantity 
- kilogram (kg), the standard SI unit of mass.
- tonne (t), a non-SI but an accepted Metric unit, defined as 1,000 kilograms.
- "short ton" is used in the USA; 1 short ton = 2,000 pounds = 0.907 tonnes.
- 1 t = (1/0.907) short tons = 1.102 short tons.
- kilogram-kilometer or kgkm, moving 1 kg of cargo a distance of 1 km;
- tonne-kilometre or tkm; 1 tkm = 1,000 kgkm;
- ton-mile (or tmi ?) in the USA: 1 ton-mile * ( 0.907 t / ton) * ( 1.609 km / mile ) = 1.459 tkm
- Kilometre-ton (unit: kmt) — the transportation of one metric ton over 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 quantity of 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 41,186 tkm).
Units of Transportation Traffic 
The transportation traffic can be defined as a measure derived from the transportation quantity, with the dimension of transportation quantity divided by unit time. Often hidden as the period of totalisation, the figures appear in statistics are often quantity per unit time, say "pkm / year" or "tmi / month" . Due to the differences and variation in the lengths of months, quarters, and years, there is some ambiguity.
Units of Transportation Density 
The 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.
See also 
- "vehicle-km". Environmental Terminology Discovery Service. European Environment Agency. Retrieved 2008-10-06.
- Duden - Wörterbuch der Abkürzungen. Von Josef Werlin. 4., neu bearbeitete und erweiterte Auflage. Mannheim, Leipzig, Wien, Zürich: Dudenverlag 1999; http://www.duden.de/duden-suche/werke/abklex/000/012/kmt.12117.html
- Woxikon - http://abkuerzungen.woxikon.de/abkuerzung/kmt.php
Here is an example of transportation statistics.