Illinois passed a law requiring all commercial trucks passing through its borders to have curved mudguards on their rear wheels. In contrast, Arkansas and other states required trucks passing through their borders to have straight mudguards. Navajo Freight Lines, Inc. (plaintiff) was an interstate commercial trucking company that challenged the Illinois law enforced by Bibb, Director of Public Safety of Illinois (defendant), on the grounds that the Illinois law was unconstitutional because changing mudguards according to state law constituted an excessive burden on interstate commerce. Navajo brought suit in district court, which held the Illinois law was an unconstitutional burden on interstate commerce. Bibb appealed to the United States Supreme Court.