359 U.S. 520 (1959)
In 1957 the State of Illinois enacted a statute, effective July 8, 1957, that required trucks and trailers to be equipped with contour rear fender mudguards.1 The inside surface must be relatively parallel to the top 90 degrees of the rear 180 degrees of the whole surface.2 The guard must extend downward to within 10 inches of the ground when the vehicle is loaded to maximum legal capacity.3 It must be wide enough to cover the tire, installed no more than 6 inches from the tire, and fitted with an outer lip or flange of at least 2 inches.4
Appellees, interstate motor carriers holding certificates from the Interstate Commerce Commission, brought suit in federal district court challenging the statute.5 A specially constituted three-judge District Court found that the Illinois requirements made the conventional straight mudflap, which was legal in at least 45 states, illegal in Illinois.6 When combined with an Arkansas Commerce Commission rule requiring straight mudflaps, the statute rendered use of the same equipment in both states impossible.7 The District Court declared the statute violative of the Commerce Clause and enjoined its enforcement.8 The Supreme Court noted probable jurisdiction on direct appeal.9
The District Court found that carriers operating into or through Illinois would have to equip all their trailers with contour mudguards because they could not determine in advance which equipment would enter the state.10 Installation cost $30 or more per vehicle.11 This produced total initial outlays for the appellees ranging from $4,500 to $45,840, with substantial additional maintenance and replacement expenses.12 Over 60 percent of the business of five of the six plaintiffs consisted of interline traffic.13 The remaining plaintiff derived 30 percent of its business from such operations.14 None of the plaintiffs operated more than 7 percent of its total annual mileage in Illinois.15
An Arkansas Commerce Commission rule required straight or conventional mudflaps.16 Trailers fitted with Illinois contour guards would not satisfy Arkansas standards, and vice versa.17 Changing between the two types of guards required two to four hours of labor.18 The contour guard is welded to the trailer, creating extreme danger when the trailer carries explosives under seal for the United States Government.19 The statute therefore threatened to disrupt interline operations essential for the movement of perishables and sealed explosives, because carriers meeting Illinois requirements could not compel originating carriers to install contour guards.20
Illinois introduced evidence that contour mudguards reduced the throwing of debris into the faces of passing drivers and the windshields of following vehicles.21 The District Court found, however, that the contour flap possessed no advantages over the conventional straight flap and created previously unknown hazards.22 These included accumulation of heat in the brake drum that decreased braking effectiveness and susceptibility to being hit and falling off when trucks were backed up.23
Whether an Illinois statute requiring the use of contour rear fender mudguards on trucks and trailers operated on the highways of that State conflicts with the Commerce Clause of the Constitution?24
States possess broad power to enact nondiscriminatory highway safety regulations applicable to interstate commerce.25 Such a measure violates the Commerce Clause when the burden it imposes on interstate commerce is clearly excessive in relation to the putative local benefits.26 This is especially true where the regulation conflicts with those of other states and disrupts interline operations essential to motor carriers.27
Yes. The Illinois statute requires contour mudguards illegal in at least 45 states.28 It forces carriers operating into or through Illinois to equip all trailers because they cannot determine in advance which equipment will enter the state.29 The statute conflicts with an Arkansas rule requiring straight mudflaps.30
This necessitates two to four hours of labor to interchange guards.31 It creates extreme danger when welding contour guards onto trailers carrying explosives under seal.32 This burden disrupts interline operations constituting over 60 percent of the business of five of the six plaintiffs.33 The District Court found that contour flaps possess no advantages over conventional flaps and create new hazards including accumulation of heat in brake drums that decreases braking effectiveness.34
The Illinois statute conflicts with the Commerce Clause.35
Related opinions on this issue
Joined by Mr. Justice Stewart
Justice Harlan, joined by Justice Stewart, concurred in the judgment of the Court.36 He noted that the opinion of the Court clearly demonstrates the heavy burden in terms of cost and interference with interlining which the Illinois statute imposes on interstate commerce.37 In view of the District Court findings that the contour mudflap possesses no advantages in terms of safety over the conventional flap permitted in all other states, this heavy burden cannot be justified.38
The findings also show that the contour flap creates previously unknown safety hazards.39 Accordingly, the burden on interstate commerce is not outweighed by any local safety benefits, and Harlan concurred in the judgment.40