In the case of Bibb, Director, Department of Public Safety of Illinois v. Navajo Freight Lines Inc., 1958, the U.S. Supreme Court ruled that an Illinois law requiring trucks to use a specific type of mudguard was unconstitutional because it violated the Commerce Clause by placing an undue burden on interstate commerce. The court found that while states have power to regulate matters concerning public safety within their borders, they cannot do so in a way that discriminates against or excessively burdens interstate commerce without justification. In this instance, there were other types of mudguards available which were equally effective and less costly for trucking companies operating across state lines to implement; thus making the requirement unreasonable and discriminatory towards out-of-state businesses.

In the dissenting opinion for Bibb v. Navajo Freight Lines, Inc., Justice Frankfurter argued that Illinois had a legitimate interest in promoting highway safety and could therefore require trucks to use a particular type of mudguard. He believed that the Court majority was too quick to dismiss this state interest and overstepped its role by substituting its own judgment about what constituted safe equipment for vehicles on highways instead of deferring to the expertise of local officials. Furthermore, he pointed out that there was no clear evidence showing these regulations were discriminatory or protectionist in nature; they applied equally to all trucks operating within Illinois regardless of their origin or destination. Therefore, according to him, it did not violate the Commerce Clause which prohibits states from enacting laws that unduly burden interstate commerce.