Mobility refers to the ease that a passenger or a freight unit can move across a transportation system. High mobility requires limited efforts, while low mobility is related to complexity and high costs. The mobility of passengers is constant in its requirements (unless involving people with disabilities), with parameters related to tolerance to acceleration and deceleration, basic comfort, air pressure, and ambient temperature (for air transport). For freight, mobility is cargo-dependent, with some commodities having limited storage requirements but heavy to carry. There are four major factors influencing mobility:
- Weight. A simple indicator of the amount of energy that must be spent to carry the cargo. Heavy cargo requires more energy to be transported as well as heavier modes and equipment to handle it. Commodities such as coal, grain, and petroleum have a high density (weight to volume ratio).
- Storage. The complexity related to holding the cargo in inventory before it can be used. This can range from simple piling to complex temperature-controlled warehousing. Commodities tend to have simple storage requirements, while retail goods need to be handled in distribution centers. At the end of the spectrum, many food and pharmaceutical products require complex storage and procedures.
- Fragility. The ease at which the cargo can be damaged during transport. Fragile cargo requires additional handling and storage procedures.
- Perishable. Some cargo degrades after being harvested or manufactured. After a specific duration, their commercial value declines or becomes negligible. Perishable cargo has a much lower mobility than cargo that does not degrade with time.
All these factors must be considered when transporting cargo since they influence the transport mode to be used and its commercial potential.