Transport supply is commonly assessed in terms of the capacity of infrastructures. Although this dimension is suitable in a number of circumstances, particularly when a single mode is considered, the concept of transport supply must consider circumstances where several modes share the same infrastructure or where intermodalism is the core factor of transport supply.
- Modal competition. In modal competition, two modes (A and B) share the same transport infrastructures. Assuming that infrastructures are used to their full capacity, the total capacity (C) is a function of their common demand, T(A) plus T(B). This common demand is the outcome of the respective modal assignment, the shares of modes A and B. In this zero-sum environment, any change in the share of one mode will be done at the expense of the other. This issue is particularly prevalent at transport bottlenecks such as bridges where different modes, namely cars and trucks, are competing for the same fixed capacity, which often results in high congestion levels.
- Intermodal capacity. The capacity of two connected intermodal terminals is derived from the terminal with the lowest capacity. Two terminals (A and B) have respective capacities of C(A) and C(B). The maximum transport demand possible between them, T(AB), is equal to the capacity of the smallest terminal, C(B). This assumes that no other activity is taking place between other terminals. In reality, an intermodal terminal is connected with several other intermodal terminals, implying that the capacity is even lower. This issue is particularly prevalent for large port and airport facilities running close to capacity. Any additional service must be carefully considered in relation to the existing capacity.