Bibb v. Navajo Freight Lines

The attorneys for Illinois did not attempt to rebut claims that the law burdened interstate commerce. Instead they argued that contoured mudguards increased safety. Specifically contoured mudguards "prevented the throwing of debris into the faces of drivers of passing cars and into the windshields of a following vehicle." However, Justice Douglas noted, these safety advantages had not been proved. Moreover, contoured mudguards created new safety hazards not found when straight mudguards were used.

Justice Douglas concluded that this was one of the rare cases where a state safety measure was unconstitutional even though it did not discriminate against persons from other states.

We deal not with absolutes but with questions of degree. The state legislatures plainly have great leeway in providing safety regulations for all vehicles--interstate as well as local . . . Yet the heavy burden which the Illinois mudguard law places on the interstate movement of trucks and trailers . . . [goes beyond] the permissible limits even for safety regulations.