Source: Panama Canal Authority (Autoridad del Canal de Panama). For fiscal years (October to September). A Panama Canal ton is the equivalent of 100 cubic feet of cargo space.
From the opening of the Panama Canal in 1914 to the early 1980s, the number of transits increased in proportion with the tonnage. This process was impacted by economic and political disruptions such as the Great Depression (1930-1936) and World War II (1941-1945). During that phase, traffic was closely related to the dynamics of the American economy since most of it was related to trade between the East and the West coast or trade between the United States and Latin America.
From the 1980s, a new dynamic was set in which the number of transits declined and stabilized around the 13,000-14,000 range. A share of the global fleet was being optimized to the Panamax standard, so as the number of transits stabilized, the tonnage gradually increased. Economies of scale were at play but constrained by what could physically fit into the locks.
Traffic particularly surged in the 2000s with growing exports from China and the usage of the all-water-route to the East Coast by shipping lines. Labor issues at American West Coast ports, such as the 2002 strike, were also an incentive to consider shipping alternatives relying more on the use of the canal. Bigger ships transiting the canal, such as containerships specifically designed to optimize cargo loads considering the dimensions of the Panama Canal locks (ships dubbed Panamax Max), became the norm. Yet, the Panama Canal had size limitations, and the growth of the tonnage in the 2000s incited the decision to expand with a new set of locks able to handle much larger ships.
After the 2016 expansion, economies of scale were quickly realized, and tonnage surged, while the number of transits did not change in a significant manner. The number of Panamax ships transits decreased, and NeoPanamax ships with about three times the capacity could transit the canal. The global shipping industry is transitioning when possible away from the Panamax standard to take advantage of the economies of scale the NeoPananamax standard confers.