Bibb v. Navajo Freight Lines

By requiring unusual mudguards, the Illinois law placed a heavy burden on interstate commerce. Only Illinois mandated contoured mudguards. Straight mudguards were required in Arkansas and permitted in at least 45 other states.

Substantial and repeated costs would be incurred to install, maintain, and replace the contoured mudguards. Since the two states required different types of mudguards, the cost would be especially high for vehicles operating in both Illinois and Arkansas. These trailers would have to stop to change mudguards every time they crossed the state border.

In addition, the law would seriously interfere with the trucking companies' "interline operations." Using this procedure, two carriers interchanged entire trailers rather than unloading and reloading the cargo. Interlining was particularly important in shipping perishable goods and explosives carried under seal. A firm not operating in Illinois would not equip its trailers with contoured mudguards. Thus its trailers could not be used by Illinois companies under the "interlining" procedure.