In the 1944 case United States et al. v. Hancock Truck Lines, Inc., the Supreme Court ruled on a matter involving interstate commerce and state taxation. The State of Michigan had imposed a tax on trucks from other states that were transporting goods through Michigan but not making any pickups or deliveries within the state. Hancock Truck Lines, an Ohio-based company, challenged this tax as unconstitutional under the Commerce Clause of the U.S Constitution which gives Congress exclusive power to regulate interstate commerce. The Supreme Court sided with Hancock Truck Lines in a unanimous decision stating that while states have some authority to impose taxes related to highway use by out-of-state vehicles, they cannot discriminate against interstate commerce by imposing higher fees or taxes than those applied to in-state businesses conducting similar activities. This ruling affirmed that such discriminatory practices are indeed violations of the Commerce Clause because they interfere with free trade among states.

In the dissenting opinion for the United States v. Hancock Truck Lines, Inc., case, it was argued that Congress had not intended to give state commissions power over interstate commerce when they enacted the Motor Carrier Act of 1935. The justices believed that this interpretation would lead to a chaotic situation where each state could impose its own regulations on interstate carriers without regard for federal law or coordination with other states. They also disagreed with the majority's view that Michigan's denial of Hancock’s application did not constitute an undue burden on interstate commerce because there were alternative routes available. The dissenters felt this reasoning ignored practical realities and economic factors such as increased costs and inefficiencies associated with longer routes.