## Facts

- Illinois enacted a highway-safety statute requiring certain trucks and trailers operating outside municipalities to use a contoured, curved rear “splash guard” (a “bubble” mudguard) meeting detailed specifications.
- Interstate motor carriers operating under federal certificates regularly ran trucks and trailers through Illinois and other states, including Arkansas.
- Arkansas required a different mudguard design (a straight, conventional flap), making the Illinois-required guard unlawful in Arkansas.
- The Illinois requirement differed from the practice permitted in most other states, creating a unique equipment standard for vehicles engaged in multi-state operations.
- The evidentiary record showed substantial operational disruption and cost for carriers, including interference with trailer interchange (“interline”) operations and the need to change or retrofit equipment at the Illinois border; the work could require welding and, in some instances, unloading and reloading.
- Illinois justified the statute as a safety measure to reduce spray, stones, and debris thrown from rear wheels.
- The record did not show a clear safety advantage for the contoured guard over conventional mudflaps and suggested possible added hazards (including heat and debris accumulation).

## Issues

1. Whether a facially nondiscriminatory Illinois highway-safety regulation requiring a unique mudguard design violates the Commerce Clause when it conflicts with another state’s requirement and materially disrupts interstate trucking operations.
2. Whether the statute’s asserted safety benefits were sufficient to justify the statute’s demonstrated burdens on interstate commerce in the absence of federal regulation.

## Decision

- The Supreme Court affirmed the injunction against enforcement of the Illinois statute.
- The Court held the statute unconstitutional as applied to interstate motor carriers because it imposed serious burdens on interstate commerce not justified by compelling safety advantages.
- The Court treated the case as an exceptional instance where a state safety measure, though within the police power and not discriminatory on its face, must yield to the national interest in an unobstructed flow of interstate commerce.

## Legal Principles

- State highway-safety regulations carry a strong presumption of validity, and courts generally defer to legislative judgments where multiple reasonable approaches could address a safety concern.
- Under the dormant Commerce Clause, a nondiscriminatory safety regulation may be invalid if, on the full record, its safety benefits are slight or uncertain while its practical effects seriously impede interstate commerce.
- Conflicting and idiosyncratic equipment requirements that force interstate carriers to reroute, duplicate equipment, or repeatedly modify vehicles can constitute an undue burden on interstate commerce, particularly when the state’s claimed safety gains are not clearly demonstrated.

## Conclusion

Illinois could not enforce a unique contoured-mudguard requirement against interstate carriers because the regulation substantially interfered with interstate trucking operations and conflicted with other states’ equipment rules, while offering only minimal and uncertain safety benefits, thereby violating the Commerce Clause.


Bibb v. Navajo Freight Lines, Inc., 359 U.S. 520 (1959)