Justice Powell
announced the judgment of the Court
and delivered an opinion, in which Justice White, Justice Blackmun, and Justice Stevens joined.
The question is whether an Iowa statute that prohibits the use of certain large trucks within the State unconstitutionally burdens interstate commerce.
I
Appellee Consolidated Freightways Corporation of Delaware (Consolidated) is one of the largest common carriers in the country. It offers service in 48 States under a certificate of public convenience and necessity issued by the Interstate Commerce Commission. Among other routes, Consolidated carries commodities through Iowa on Interstate 80, the principal east-west route linking New York, Chicago, and the west coast, and on Interstate 35, a major north-south route.
Consolidated mainly uses two kinds of trucks. One consists of a three-axle tractor pulling a 40-foot two-axle trailer. This unit, commonly called a single, or “semi,” is 55 feet in length overall. Such trucks have long been used on the Nation’s highways. Consolidated also uses a two-axle tractor pulling a single-axle trailer which, in turn, pulls a single-axle dolly and a second single-axle trailer. This combination, known as a double, or twin, is 65 feet long overall. Many trucking companies, including Consolidated, increasingly prefer to use doubles to ship certain kinds of commodities. Doubles have larger capacities, and the trailers can be detached and routed separately if necessary. Consolidated would like to use 65-foot doubles on many of its trips through Iowa.
The State of Iowa, however, by statute restricts the length of vehicles that may use its highways. Unlike all other States in the West and Midwest, App. 605, Iowa generally prohibits the use of 65-foot doubles within its borders. Instead, most truck combinations are restricted to 55 feet in length. Doubles, mobile homes, trucks carrying vehicles such as tractors and other farm equipment, and singles hauling livestock, are permitted to be as long as 60 feet. Notwithstanding these restrictions, Iowa’s statute permits cities abutting the state line by local ordinance to adopt the length limitations of the adjoining State. Iowa Code § 321.467 (7) (1979). Where a city has exercised this option, otherwise oversized trucks are permitted within the city limits and in nearby commercial zones. Ibid.
Iowa also provides for two other relevant exemptions. An Iowa truck manufacturer may obtain a permit to ship trucks that are as large as 70 feet. Iowa Code § 321E.10 (1979). Permits also are available to move oversized mobile homes, provided that the unit is to be moved from a point within Iowa or delivered for an Iowa resident. § 321E.28 (5).
Because of Iowa’s statutory scheme, Consolidated cannot use its 65-foot doubles to move commodities through the State. Instead, the company must do one of four things: (i) use 55-foot singles; (ii) use 60-foot doubles; (iii) detach the trailers of a 65-foot double and shuttle each through the State separately; or (iv) divert 65-foot doubles around Iowa.
Dissatisfied with these options, Consolidated filed this suit in the District Court averring that Iowa’s statutory scheme unconstitutionally burdens interstate commerce. Iowa defended the law as a reasonable safety measure enacted pursuant to its police power. The State asserted that 65-foot doubles are more dangerous than 55-foot singles and, in any event, that the law promotes safety and reduces road wear within the State by diverting much truck traffic to other States.
In a 14-day trial, both sides adduced evidence on safety, and on the burden on interstate commerce imposed by Iowa’s law. On the question of safety, the District Court found that the “evidence clearly establishes that the twin is as safe as the semi.’’ 475 F. Supp. 544, 549 (SD Iowa 1979). For that reason,
“there is no valid safety reason for barring twins from Iowa’s highways because of their configuration.
“The evidence convincingly, if not overwhelmingly, establishes that the 65 foot twin is as safe as, if not safer than, the 60 foot twin and the 55 foot semi. . . .
“Twins and semis have different characteristics. Twins are more maneuverable, are less sensitive to wind, and create less splash and spray. However, they are more likely than semis to jackknife or upset. They can be backed only for a short distance. The negative characteristics are not such that they render the twin less safe than semis overall. Semis are more stable but are more likely to hear end’ another vehicle.” Id., at 548-549.
In light of these findings, the District Court applied the standard we enunciated in Raymond Motor Transportation, Inc. v. Rice, 434 U. S. 429 (1978), and concluded that the state law impermissibly burdened interstate commerce:
“[T]he balance here must be struck in favor of the federal interests. The total effect of the law as a safety measure in reducing accidents and casualties is so slight and problematical that it does not outweigh the national interest in keeping interstate commerce free from interferences that seriously impede it.” 475 F. Supp., at 551 (emphasis in original).
The Court of Appeals for the Eighth Circuit affirmed. 612 F. 2d 1064 (1979). It accepted the District Court's finding that 65-foot doubles were as safe as 55-foot singles. Id., at 1069. Thus, the only apparent safety benefit to Iowa was that resulting from forcing large trucks to detour around the State, thereby reducing overall truck traffic on Iowa’s highways. The Court of Appeals noted that this was not a constitutionally permissible interest. Id., at 1070. It also commented that the several statutory exemptions identified above, such as those applicable to border cities and the shipment of livestock, suggested that the law in effect benefited Iowa residents at the expense of interstate traffic. Id., at 1070-1071. The combination of these exemptions weakened the presumption of validity normally accorded a state safety regulation. For these reasons, the Court of Appeals agreed with the District Court that the Iowa statute unconstitutionally burdened interstate commerce.
Iowa appealed, and we noted probable jurisdiction. 446 U. S. 950 (1980). We now affirm.
II
It is unnecessary to review in detail the evolution of the principles of Commerce Clause adjudicátion. The Clause is both a “prolific sourcfe] of national power and an equally prolific source of conflict with legislation of the state [s].” H. P. Hood & Sons, Inc. v. Du Mond, 336 U. S. 525, 534 (1949). The Clause permits Congress to legislate when it perceives that the national welfare is not furthered by the independent actions of the States. It is now well established, also, that the Clause itself is “a limitation upon state power even without congressional implementation.” Hunt v. Washington Apple Advertising Comm’n, 432 U. S. 333, 350 (1977). The Clause requires that some aspects of trade generally must remain free from interference by the States. When a State ventures excessively into the regulation of these aspects of commerce, it “trespasses upon national interests,” Great A&P Tea Co. v. Cottrell, 424 U. S. 366, 373 (1976), and the courts will hold the state regulation invalid under the Clause alone.
The Commerce Clause does not, of course, invalidate all state restrictions on commerce. It has long been recognized that, “in the absence of conflicting legislation by Congress, there is a residuum of power in the state to make laws governing matters of local concern which nevertheless in some measure affect interstate commerce or even, to some extent, regulate it.” Southern Pacific Co. v. Arizona, 325 U. S. 761, 767 (1945). The extent of permissible state regulation is not always easy to measure. It may be said with confidence, however, that a State’s power to regulate commerce is never greater than in matters traditionally of local concern. Washington Apple Advertising Comm’n, supra, at 350. For ex-. ample, regulations that touch upon safety — especially highway safety — are those that “the Court has been most reluctant to invalidate.” Raymond, supra, at 443; accord, Railway Express Agency, Inc. v. New York, 336 U. S. 106, 109 (1949); South Carolina State Highway Dept. v. Barnwell Brothers, Inc., 303 U. S. 177, 187 (1938); Sproles v. Binford, 286 U. S. 374, 390 (1932); Hendrick v. Maryland, 235 U. S. 610, 622 (1915). Indeed, “if safety justifications are not illusory, the Court will not second-guess legislative judgment about their importance in comparison with related burdens on interstate commerce.” Raymond, supra, at 449 (Blackmun, J., concurring). Those who would challenge such bona fide safety regulations must overcome a “strong presumption of validity.” Bibb v. Navajo Freight Lines, Inc., 359 U. 520, 524 (1959).
But the incantation of a purpose to promote the public health or safety does not insulate a state law from Commerce Clause attack. Regulations designed for that salutary purpose nevertheless may further the purpose so marginally, and interfere with commerce so substantially, as to be invalid under the Commerce Clause. In the Court’s recent unanimous decision in Raymond, we declined to “accept the State’s contention that the inquiry under the Commerce Clause is ended without a weighing of the asserted safety purpose against the degree of interference with interstate commerce.” 434 U. S., at 443. This “weighing” by a court requires — and indeed the constitutionality of the state regulation depends on — “a sensitive consideration of the weight and nature of the state regulatory concern in light of the extent of the burden imposed on the course of interstate commerce.” Id., at 441; accord, Pike v. Bruce Church, Inc., 397 U. S. 137, 142 (1970); Bibb, supra, at 525-530; Southern Pacific, supra, at 770.
Ill
Applying these general principles, we conclude that the Iowa truck-length limitations unconstitutionally burden interstate commerce.
In Raymond Motor Transportation, Inc. v. Rice, the Court held that a Wisconsin statute that precluded the use of 65-foot doubles violated the Commerce Clause. This case is Raymond revisited. Here, as in Raymond, the State failed to present any persuasive evidence that 65-foot doubles are less safe than 55-foot singles. Moreover, Iowa’s law is now out of step with the laws of all other Midwestern and Western States. Iowa thus substantially burdens the interstate flow of goods by truck. In the absence of congressional action to set uniform standards, some burdens associated with state safety regulations must be tolerated. But where, as here, the State’s safety interest, has. been.iouml to be inüsbry, and its regulations impair significantly the federal interest in efficient and safe interstate transportation, the state law cannot be harmonized with the Commerce Clause.
A
Iowa made a more serious effort to support the safety-rationale of its .law than, did .Wisconsin in Raymond, but its effort was no more persuasive. As noted above, the District Court found that the “evidence clearly establishes that the twin is as safe as the semi.” The record supports this finding.
The trial focused on a comparison of the performance of the two kinds of trucks in various safety categories. The evidence showed, and the District Court found, that the 65-foot double was at least the equal of the 55-foot single in the ability to brake, turn, and maneuver. The double, because of its axle placement, produces less splash and spray in wet weather. And, because of its articulation in the middle, the double is less susceptible to dangerous “off-tracking,” and to wind.
None of these findings is seriously disputed by Iowa. Indeed, the State points to only three ways in which the 55-foot single is even arguably superior: singles take less time to be passed and to clear intersections; they may back up for longer distances; and they are somewhat less likely to jackknife.
The first two of these characteristics are of limited relevance on modern interstate highways. As the District Court found, the negligible difference in the' time required to pass, and to cross intersections, is insignificant on 4-lane divided highways because passing does not require crossing into oncoming traffic lanes, Raymond, 434 U. S., at 444, and interstates have few, if any, intersections. The concern over backing capability also is insignificant because it seldom is necessary to back up on an interstate. In any event, no evidence suggested any difference in backing capability between the 60-foot doubles that Iowa permits and the 65-foot doubles that it bans. Similarly, although doubles tend to jackknife somewhat more than singles, 65-foot doubles actually are less likely to jackknife than 60-foot doubles.
Statistical studies supported the view that 65-foot doubles are at least as safe overall as 55-foot singles and 60-foot doubles. One such study, which the District Court credited, reviewed Consolidated’s comparative accident experience in 1978 with its own singles and doubles. Each kind of truck was driven 56 million miles on identical routes. The singles were involved in 100 accidents resulting in 27 injuries and one fatality. The 65-foot doubles were involved in 106 accidents resulting in 17 injuries and one fatality. Iowa’s expert statistician admitted that this study provided “moderately strong evidence” that singles have a higher injury rate than doubles. App. 488. Another study, prepared by the Iowa Department of Transportation at the request of the state legislature, concluded that “[s]ixty-five foot twin trailer combinations have not been shown by experiences in other states to be less safe than 60 foot twin trailer combinations or conventional tractor-semitrailers” (emphasis in original). Id., at 584. Numerous insurance company executives, and transportation officials from the Federal Government and various States, testified that 65-foot doubles were at least as safe as 55-foot singles. Iowa concedes that it can produce no study that establishes a statistically significant difference in safety between the 65-foot double and the kinds of vehicles the State permits. Brief for Appellants 28, 32. Nor, as the District Court noted, did Iowa present a single witness who testified that 65-foot doubles were more dangerous overall than the vehicles permitted under Iowa law. 475 F. Supp., at 549. In sum, although Iowa introduced more evidence on the question of safety than did Wisconsin in Raymond, the record as a whole was not more favorable to the State.
B
Consolidated, meanwhile, demonstrated that Iowa’s law substantially burdens interstate commerce. Trucking companies that wish to continue to use 65-foot doubles must route them around Iowa or detach the trailers of the doubles and ship them through separately. Alternatively, trucking companies must use the smaller 55-foot singles or 60-foot doubles permitted under Iowa law. Each of these options engenders inefficiency and added expense. The record shows that Iowa’s law added about $12.6 million each year to the costs of trucking companies. Consolidated alone incurred about $2 million per year in increased costs.
In addition to increasing the costs of the trucking companies (and, indirectly, of the service to consumers), Iowa’s law may aggravate, rather than ^meliorate, the problem of highway accidents. Fifty-five foot singles carry less freight than 65-foot doubles. Either more small trucks must be used to carry the same quantity of goods through Iowa, or the same number of larger trucks must drive longer distances to bypass Iowa. In either case, as the District Court noted, the restriction requires more highway miles to be driven to transport the same quantity of goods. Other things being equal, accidents are proportional to distance traveled. See App. 604, 615. Thus, if 65-foot doubles are as safe as 55-foot singles, Iowa’s law tends to increase the number of accidents, and to shift the incidence of them from Iowa to other States.
IV
Perhaps recognizing the weakness of the evidence supporting its safety argument, and the substantial burden on commerce that its regulations create, Iowa urges the Court simply to “defer” to the safety judgment of the State. It argues that the length of trucks is generally, although perhaps imprecisely, related to safety. The task of drawing a line is one that Iowa contends should be left to its legislature.
The Court normally does accord “special deference” to state highway safety regulations. Raymond, 434 U. S., at 444, n. 18. This traditional deference “derives in part from the assumption that where such regulations do not discriminate on their face against interstate commerce, their burden usually falls on local economic interests as well as other States’ economic interests, thus insuring that a State’s own political processes will serve as a check against unduly burdensome regulations.” Ibid. Less deference to the legislative judgment is due, however, where the local regulation bears disproportionately on out-of-state residents and businesses. Such a disproportionate burden is apparent here. Iowa’s scheme, although generally banning large doubles from the State, nevertheless has several exemptions that secure to Iowans many of the benefits of large trucks while shunting to neighboring States many of the costs associated with their use.
At the time of trial there were two particularly significant exemptions. First, singles hauling livestock or farm vehicles were permitted to be as long as 60 feet. Iowa Code §§ 321.457 (5), 321.457 (3) (1979). As the Court of Appeals noted, this provision undoubtedly was helpful to local interests. Cf. Raymond, supra, at 434 (exemption in Wisconsin for milk shippers). Second, cities abutting other States were permitted to enact local ordinances adopting the larger length limitation of the neighboring State. Iowa Code § 321.457 (7) (1979). This exemption offered the benefits of longer trucks to individuals and businesses in important border cities without burdening Iowa’s highways with interstate through traffic. Cf. Raymond, supra, at 446-447, and n. 24 (exemption in Wisconsin for shipments from local plants).
The origin of the “border cities exemption” also suggests that Iowa’s statute may not have been designed to ban dangerous trucks, but rather to discourage interstate truck traffic. In 1974, the legislature passed a bill that would have permitted 65-foot doubles in the State. See n. 6, supra. Governor Ray vetoed the bill. He said:
“I find sympathy with those who are doing business in our state and whose enterprises could gain from increased cargo carrying ability by trucks. However, with this bill, the Legislature has pursued a course that would benefit only a few Iowa-based companies while providing a great advantage for out-of-state trucking firms and competitors at the expense of our Iowa citizens.” App. 626.
After the veto, the “border cities exemption” was immediately enacted and signed by the Governor.
It is thus far from clear that Iowa was motivated primarily by a judgment that 65-foot doubles are less safe than 55-foot singles. Rather, Iowa seems to have hoped to limit the use of its highways by deflecting some through traffic. In the District Court and Court of Appeals, the State explicitly at-temped to justify the law by its claimed interest in keeping trucks out of Iowa. See n. 9 and accompanying text, supra. The Court of Appeals correctly concluded that a State cannot constitutionally promote its own parochial interests by requiring safe vehicles to detour around it. 612 F. 2d, at 1070.
y
In sum, the statutory exemptions, their history, and the arguments Iowa has advanced in support of its law in this litigation, all suggest that the deference traditionally accorded a State’s safety judgment is not warranted. See Raymond, supra, at 444, and n. 18, 446-447. The controlling factors thus are the findings of the District Court, accepted by the Court of Appeals, with respect to the relative safety of the types of trucks at issue, and the substantiality of the burden on interstate commerce.
Because Iowa has imposed this burden without any significant countervailing safety interest, its statute violates the Commerce Clause. The judgment of the Court of Appeals is affirmed.
It is so ordered.
For an illustration of the differences between singles and doubles, see Raymond Motor Transportation, Inc. v. Rice, 417 F. Supp. 1352, 1363 (WD Wis. 1976) (three-judge court), rev’d, 434 U. S. 429 (1978).
Iowa Code §321.457 (6) (1979). The 60-foot double is not commonly used anywhere except in Iowa. It consists of a tractor pulling a large trailer, which in turn pulls a dolly attached to a small trailer. The odd-sized trailer used in the 60-foot double is not compatible for interchangeable use in other trailer combinations. See App. 23, 276-277, 353, 354.
Iowa Code §321.457 (4) (1979).
§321.457 (5).
§321.457 (3). After trial, and after the Court of Appeals’ decision in this case, Iowa amended its law to permit all singles to be as large as 60 feet. 1980 Iowa Acts, ch. 1100.
The Iowa Legislature in 1974 passed House Bill 671, which would have permitted 65-foot doubles. But Iowa Governor Ray vetoed the bill, noting that it “would benefit only a few Iowa-based companies while providing a great advantage for out-of-state trucking firms and competitors at the expense of our Iowa citizens.” Governor’s Veto Message of March 2, 1974, reprinted in App. 626. The “border-cities exemption” was passed by the General Assembly and signed by the Governor shortly thereafter.
The Iowa Transportation Commission, pursuant to authority conferred in Iowa Code §307.10 (5) (1979), subsequently adopted regulations that would have legalized 65-foot doubles, provided that the legislature enacted a ban on studded snow tires. The Iowa Supreme Court declared these regulations void because their promulgation was impermissibly tied to legislative action. Motor Club of Iowa v. Department of Transportation, 251 N. W. 2d 510 (1977).
The parochial restrictions in the mobile home provision were enacted after Governor Ray vetoed a bill that would have permitted the interstate shipment of all mobile homes through Iowa. Governor Ray commented, in his veto message:
“This bill . . . would make Iowa a bridge state as these oversized units are moved into Iowa after being manufactured in another state and sold in a third. None of this activity would be of particular economic benefit to Iowa.” Governor’s Veto Message of March 16, 1972, reprinted in App. 641.
Defendants, appellants in this Court, are Raymond Kassel, Director of the Iowa Department of Transportation, Iowa Governor Robert D. Ray, and state transportation officials Robert Rigler, L. Stanley Schoelerman, Donald Gardner, Jules Busker, Allan Thoms, Barbara Dunn, William McGrath, Jon McCoy, Charles W. Larson, Edward Dickinson, and Richard C. Turner.
See 475 F. Supp. 544, 551 (SD Iowa 1979); 612 F. 2d 1064, 1068, 1069-1070 (CA8 1979). In this Court, Iowa places little or no emphasis on the constitutional validity of this second argument.
Justice Stevens took no part in the consideration or decision of Raymond.
The Senate last year passed a bill that would have pre-empted the field of truck lengths by setting a national limit of 65 feet. See S. 1390, 96th Cong., 2d Sess. (1980) (reprinted in 126 Cong. Rec. 3309, 3303 (1980)). The House took no action before adjournment.
It is highly relevant that here, as in Raymond, the state statute contains exemptions that weaken the deference traditionally accorded to a state safety regulation. See Part- IV, infra.
Twin trailers have single axles; semis, by contrast, have tandem axles. The axle configuration of the semi aggravates splash and spray. The forward tire creates upward wind currents in the same place that the rear tire creates downward wind currents. The confluence of these currents occurs at a point just above and between the tandem axles. The resulting turbulence then is blasted outward, carrying spray with it. App. 95-96.
“Off-tracking” refers to the extent to which the rear wheels of a truck deviate from the path of the front wheels while turning.
Evidence at trial did show that doubles could back up far enough to move around an accident. App. 103.
In suggesting that Iowa's law actually promotes safety, the dissenting opinion ignores the findings of the courts below and relies on largely discredited statistical evidence. The dissent implies that a statistical study identified doubles as more dangerous than singles. Post, at 695. At trial, however, the author of that study — Iowa’s own statistician — conceded that his calculations were statistically biased, and therefore “not very meaningful.” Tr. 1678; see App. 669-670, Tr. 1742-1747.
The dissenting opinion also suggests that its conclusions are bolstered by the fact that the American Association of State Highway and Transportation Officials (AASHTO) recommends that States limit truck lengths. Post, at 693, 699. The dissent fails to point out, however, that AASHTO specifically recommends that States permit 65-foot doubles. App. 602-603.
Moreover, trucks diverted from interstates often must travel over more dangerous roads. For example, east-west traffic diverted from Interstate 80 is rerouted through Missouri on U. S. Highway 36, which is predominantly a 2-lane road.
The District Court, in denying a stay pending appeal, noted that Iowa’s law causes “more accidents, more injuries, more fatalities and more fuel consumption.” Id., at 579. Appellant Kassel conceded as much at trial. Id., at 281. Kassel explained, however, that most' of these additional accidents occur in States other than Iowa because truck traffic is deflected around the State. He noted: “Our primary concern is the citizens of Iowa and our own highway system we operate in this state.” Ibid.
As the District Court noted, diversion of traffic benefits Iowa by holding down (i) accidents in the State, (ii) auto insurance premiums, (iii) police staffing needs, and (iv) road wear. 475 F. Supp., at 550.
Five of Iowa’s ten largest cities — Davenport, Sioux City, Dubuque, Council Bluffs, and Clinton — are by their location entitled to use the “border cities exemption.” See U. S. Bureau of the Census, U. S. Census of Population: 1970 Number of Inhabitants, Final Report, PC (1)-A1, United States Summary 1-136, 1-137.
The vast majority of the 65-foot doubles seeking access to Iowa’s interstate highways carry goods in interstate traffic through Iowa. See App. 175-176, 560.
As noted above, exemptions also are available to benefit Iowa truck makers, Iowa Code § 321E.10 (1979), and Iowa mobile home manufacturers or purchasers, §321E.28 (5). Although these exemptions are not directly relevant to the controversy over the safety of 65-foot doubles, they do contribute to the pattern of parochialism apparent in Iowa’s statute.
Governor Ray further commented that “if we have thousands more trucks crossing our state, there will be millions of additional miles driven in Iowa and that does create a genuine concern for safety.” App. 628.
The dissenting opinion insists that we defer to Iowa’s truck-length limitations because they represent the collective judgment of the Iowa Legislature. See post, at 691-692, 696-697, 699, 700. This position is curious because, as noted above, the Iowa Legislature approved a bill legalizing 65-foot doubles. The bill was vetoed by the Governor, primarily for parochial rather than legitimate safety reasons. The dissenting opinion is at a loss to explain the Governor’s interest in deflecting interstate truck traffic around Iowa.
Locomotive Firemen v. Chicago, R. I. & P. R. Co., 393 U. S. 129 (1968), in its result, although perhaps not in all of its language, is consistent with the conclusion we reach today. There, the Arkansas “full-crew” laws were upheld against constitutional challenge because the Court easily perceived that they made nonillusory contributions to safety. See id., at 136-138. Here, as in Raymond, there was no such evidence. This case and Raymond recognize, as the Court did in Locomotive Firemen, that States constitutionally may enact laws that demonstrably promote safety, even when those laws also burden the flow of commerce.
As noted above, the District Court and the Court of Appeals held that the Iowa statutory scheme unconstitutionally burdened interstate commerce. The District Court, however, found that the statute did not discriminate against such commerce. 475 F. Supp., at 553. Because the record fully supports the decision below with respect to the burden on interstate commerce, we need not consider whether the statute also operated to discriminate against that commerce. See Raymond, 434 U. S., at 446-447, n. 24. The latter theory was neither briefed nor argued in this Court.
Justice Rehnquist in dissent states that, as he reads the various opinions in this case, “only four Justices invalidate Iowa’s law on the basis of the analysis in Raymond." Post, at 700, n. 10. It should be emphasized that Raymond, the analysis of which was derived from the Court’s opinion in Pike v. Bruce Church, Inc., 397 U. S. 137 (1970), was joined by each of the eight Justices who participated. Today, Justice Brennan finds it unnecessary to reach the Raymond analysis because he finds the Iowa statute to be flawed for a threshold reason.
Consolidated’s complaint sought only a declaration that the Iowa statute was unconstitutional insofar as it precluded the use of 65-foot doubles on major interstate highways and nearby access roads. App. 19-11. We are not asked to consider whether Iowa validly may ban 65-foot doubles from smaller roads on which they might be demonstrably unsafe.