sells 24 of these daily slots up to 365 days in advance. The second choice is high priority transit. Since 2006, ACP has made available a 25th slot, sold through the Transit Slot Auction to the highest bidder. The main customers of the Transit Booking System are cruise ships, container ships, vehicle carriers, and non-containerized cargo vessels. Cargo volume The Panama Canal Authority predicts that the volume of cargo transiting the canal will grow by an average of 3% per year, doubling the 2005 tonnage by 2025. Allowing larger vessels to transit the canal will move more cargo per transit and volume of water used. Historically, the dry and liquid bulk segments have generated most of the canal's revenues. Bulk cargo includes dry goods, such as grains (corn, soy, and wheat, among others), minerals, fertilizers, coal, and liquid goods, such as chemical products, propane gas, crude oil, and oil derivatives. Recently, containerized cargo has replaced dry bulk as the canal's main income generator, moving it to second place. Vehicle carriers have become the third-largest income generator, replacing the liquid bulk segment. Shipping industry analyses conducted by the ACP and top industry experts indicate that the canal expansion will be beneficial to both the canal and its users because of the demand that will be served by allowing the transit of more tonnage. However, the question is whether the trend on which the ACP makes those projections can continue for a generation. The growth in usage of the Panama Canal over the past few years has been almost entirely driven by increased US imports from China passing through the canal en route to ports on the US East and Gulf coasts. But it is increasingly recognized in both the US and China that this imbalance in trade is unsustainable and will be reduced via some sort of adjustment in the coming years (although such an imbalance need not be made up by physically shipped goods, but could be made by other trade such as intellectual property as China upgrades its intellectual property protection laws). The ACP, however, presumes that trade will continue to grow for a generation as it has for the past several years. One of the central points made by critics of canal expansion, most prominently former canal administrator Fernando Manfredo, is that it is unrealistic to attempt to predict canal usage trends over a generation, improbable to expect