Empty movements are among the most significant operational and commercial constraints in transportation, affecting both passengers and freight flows alike. They involve the repositioning of transport assets, which does not generate any income during the process, implying that this cost must be absorbed. For instance, about 20% of all containers are moved empty as they are being repositioned. Four main factors are explaining why several transport flows are empty:
- Imbalanced Flows. The demand for transportation between two locations is almost never balanced, but there is reciprocity in these imbalances. For instance, international trade relations underline that some nations import more than they export, while it is the opposite for others (export-oriented economies). Similar patterns can be observed at the regional or local (urban) levels. Imbalances can also have temporal implications. While at an aggregate level, flows could appear to be balanced, they can be highly imbalanced for specific time periods. For instance, over a 24-hour period, commuting would appear to involve balanced flows as each commuter returns to the place of residence. However, commuting usually involves imbalanced movements between the central area of a city and its periphery, with the dominance of inbound movements in the morning and outbound movements in the afternoon.
- Cargo and Equipment Specialization. Some types of cargo can only be transported with specialized modes and equipment. Thus, even if there is cargo available for a backhaul movement, the conveyance for the inbound haul may not be suitable. For instance, refrigerated vehicles and containers are not well designed to carry other types of cargo, implying that cold chain transportation usually has empty backhauls. Further, it is common in many bulk trades, such as petroleum, grains, and minerals, to have an empty backhaul because the ships are specialized carriers not designed to carry anything else. Containerization allows carrying a wide variety of cargo, being less prone to imbalances.
- Short Hauls. Many transport flows are over short distances and cover a specific sequence, such as feeders or local deliveries. They may thus be unable to be available for backhaul cargo opportunities that could be further away. As such, the range of transport services imposes limitations on their market opportunities and the availability of backhaul flows.
- Regulatory Constraints. Although there could be opportunities for backhaul movements, the regulatory context may prevent them. For instance, cabotage regulations prevent maritime shipping companies, airlines, and trucking companies from carrying cargo or passengers in many national markets. A foreign-flagged shipping company cannot carry cargo between two American ports, and an international airline cannot carry passengers between two American airports. In the taxi industry, a driver can only pick up customers in specific jurisdictions (e.g. a town or a city). Own account transporters (e.g. the transport branch of a large retailer) can only carry cargo for the corporation they are part of, often resulting in more empty hauls.
There is limited effective mitigation to empty movements because many are the outcome of macroeconomic or spatial processes that cannot be readily influenced in the short term. Trade imbalances are mainly the outcome of differences in comparative advantages, such as labor costs or the availability of resources. Containerization has enabled a better mix of cargo, but the large commodity trades still depend on specialized conveyances. Regulatory changes, such as cabotage restrictions, could be alleviated, but national or regional markets are usually protected by special interest groups unwilling to forego the rent-seeking advantages that this implies.