Inc. South Carolina State Highway Department v. Barnwell Bros.

After South Carolina State Highway Department, the Supreme Court was not always so sympathetic to the states regarding the Commerce Clause. In general, the Court gave the states less leeway in regulating railways versus highways. In Southern Pacific Co. v. Arizona (1945) the Court struck down a state law limiting the number of cars on trains traveling through Arizona. In that case, the Court said the safety issue involved did not outweigh the burden on interstate commerce.

Gradually, the Court also limited the states' power over regulating highways. It struck down an Illinois law that required a specific type of mud flap for trucks using state highways. In Bibb v. Navajo Freight Lines (1959), the Court said the state law was not discriminatory, but its safety intent did place an unreasonable burden on interstate commerce. Later decisions struck down state laws prohibiting double-trailer trucks and other oversized trucks on state highways.