with whom Justice Marshall joins, concurring in the judgment.
Iowa’s truck-length regulation challenged in this case is nearly identical to the Wisconsin regulation struck down in Raymond Motor Transportation, Inc. v. Rice, 434 U. S. 429 (1978), as in violation of the Commerce Clause. In my view the same Commerce Clause restrictions that dictated that holding also require invalidation of Iowa’s regulation insofar as it prohibits 65-foot doubles.
The reasoning bringing me to that conclusion does not require, however, that I engage in the debate between my Brothers Powell and Rehnquist over what the District Court record shows on the question whether 65-foot doubles are more dangerous than shorter trucks. With all respect, my Brothers ask and answer the wrong question.
For me, analysis of Commerce Clause challenges to state regulations must take into account three principles: (1) The courts are not empowered to second-guess the empirical judgments of lawmakers concerning the utility of legislation. *680(2) The burdens imposed on commerce must be balanced against the local benefits actually sought to be achieved by the State’s lawmakers, and not against those suggested after the fact by counsel. (3) Protectionist legislation is unconstitutional under the Commerce Clause, even if the burdens and benefits are related to safety rather than economics.
I
Both the opinion of my Brother Powell and the opinion of my Brother Rehnquist are predicated upon the supposition that the constitutionality of a state regulation is determined by the factual record created by the State’s lawyers in trial court. But that supposition cannot be correct, for it would make the constitutionality of state laws and regulations depend on the vagaries of litigation rather than on the judgments made by the State’s lawmakers.
In considering a Commerce Clause challenge to a state regulation, the judicial task is to balance the burden imposed on commerce against the local benefits sought to be achieved by the State’s lawmakers. See Pike v. Bruce Church, Inc., 397 U. S. 137, 142 (1970). In determining those benefits, a court should focus ultimately on the regulatory purposes identified by the lawmakers and on the evidence before or available to them that might have supported their judgment. See generally Minnesota v. Clover Leaf Creamery Co., 449 U. S. 456, 464, 473 (1981). Since the court must confine its analysis to the purposes the lawmakers had for maintaining the regulation, the only relevant evidence concerns whether the lawmakers could rationally have believed that the challenged regulation would foster those purposes. See Locomotive Firemen v. Chicago, R. I. & P. R. Co., 393 U. S. 129, 138-139 (1968); South Carolina State Highway Dept. v. Barnwell Bros., Inc., 303 U. S. 177, 192-193 (1938). It is not the function of the court to decide whether in fact the regulation promotes its intended purpose, so long as an examination of the evidence before or available to the lawmaker indicates *681that the regulation is not wholly irrational in light of its purposes. See Minnesota v. Clover Leaf Creamery Co., supra, at 469, 473.1
II
My Brothers Powell and REHjisrQuiST make the mistake of disregarding the intention of Iowa’s lawmakers and assuming that resolution of the case must hinge upon the argument offered by Iowa’s attorneys: that 65-foot doubles are more dangerous than shorter trucks. They then canvass the factual record and findings of the courts below and reach opposite conclusions as to whether the evidence adequately supports that empirical judgment. I repeat: my Brothers Powell and Rehnquist have asked and answered the wrong question. For although Iowa’s lawyers in this litigation have defended the truck-length regulation on the basis of the safety advantages of 55-foot singles and 60-foot doubles over 65-foot doubles, Iowa’s actual rationale for maintaining the regulation had nothing to do with these purported differences. Rather, Iowa sought to discourage interstate truck traffic on Iowa’s high*682ways.2 Thus, the safety advantages and disadvantages of the types and lengths of trucks involved in this case are irrelevant to the decision.3
*683My Brother Powell concedes that “[i]t is . . . 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 trafile.” Ante, at 677. This conclusion is more than amply supported by the record and the legislative history of the Iowa regulation. The Iowa Legislature has consistently taken the position that size, weight, and speed restrictions on interstate trafile should be set in accordance with uniform national standards. The stated purpose was not to further safety but to achieve uniformity with other States. The Act setting the limitations challenged in *684this case, passed in 1947 and periodically amended since then, is entitled “An Act to promote uniformity with other states in the matter of limitations on the size, weight and speed of motor vehicles . . . 1947 Iowa Acts, ch. 177 (emphasis added). Following the proposals of the American Association of State Highway and Transportation Officials, the State has gradually increased the permissible length of trucks from 45 feet in 1947 to the present limit of 60 feet.
In 1974, the Iowa Legislature again voted to increase the permissible length of trucks to conform to uniform standards then in effect in most other States. This legislation, House Bill 671, would have increased the maximum length of twin trailer trucks operable in Iowa from 60 to 65 feet. But Governor Bay broke from prior state policy, and vetoed the legislation. The legislature did not override the veto, and the present regulation was thus maintained. In his veto,4 Governor Kay did not rest his decision on the conclusion that 55-foot singles and 60-foot doubles are any safer than 65-foot doubles, or on any other safety consideration inherent in the type or size of the trucks. Rather, his principal concern was that to allow 65-foot doubles would “basically ope[n] our state to literally thousands and thousands more trucks per year.” App. 628. This increase in interstate truck traffic would, in the Governor’s estimation, greatly increase highway maintenance costs, which are borne by the citizens of the State, id., at 628-629, and increase the number of accidents and fatalities within the State. Id., at 628. The legislative response was not to override the veto, but to accede to the Governor’s action, and in accord with his basic premise, to enact a “border cities exemption.” This permitted cities within border areas to allow 65-foot doubles while otherwise maintaining the 60-foot limit throughout the State to discourage interstate truck traffic.
*685Although the Court has stated that “[i]n no field has . . . deference to state regulation been greater than that of highway safety,” Raymond Motor Transportation, Inc. v. Rice, 434 U. S., at 443, it has declined to go so far as to presume that size restrictions are inherently tied to public safety. Id., at 444, n. 19. The Court has emphasized that the “strong presumption of validity” of size restrictions “cannot justify a court in closing its eyes to uncontroverted evidence of record,” ibid. — here the obvious fact that the safety characteristics of 65-foot doubles' did not provide the motivation for either legislators or Governor in maintaining the regulation.
Ill
Though my Brother Powell recognizes that the State’s actual purpose in maintaining the truck-length regulation was “to limit the use of its highways by deflecting some through traffic,” ante, at 677, he fails to recognize that this purpose, being protectionist in nature, is impermissible under the Commerce Clause.5 The Governor admitted that he blocked legislative efforts to raise the length of trucks because the change “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; see also id., at 185-186. Appellant Raymond Kassel, Director of the Iowa Department of Transportation, while admitting that the greater 65-foot length standard would be safer overall, defended the more restrictive regulations because of their benefits within Iowa:
“Q: Overall, there would be fewer miles of operation, fewer accidents and fewer fatalities?
“A: Yes, on the national scene.
“Q: Does it not concern the Iowa Department of *686Transportation that banning 65-foot twins causes more accidents, more injuries and more fatalities?
“A: Do you mean outside of our state border?
“Q: Overall.
“A: Our primary concern is the citizens of Iowa and our own highway system we operate in this state.” Id., at 281.
The regulation has had its predicted effect. As the District Court found:
“Iowa’s length restriction causes the trucks affected by the ban to travel more miles over more dangerous roads in other states which means a greater overall exposure to accidents and fatalities. More miles of highway are subjected to wear. More fuel is consumed and greater transportation costs are incurred.” 475 F. Supp. 544, 550 (SD Iowa 1979).
Iowa may not shunt off its fair share of the burden of maintaining interstate truck routes, nor may it create increased hazards on the highways of neighboring States in order to decrease the hazards on Iowa highways. Such an attempt has all the hallmarks of the “simple . . . protectionism” this Court has condemned in the economic area. Philadelphia v. New Jersey, 437 U. S. 617, 624 (1978). Just as a State’s attempt to avoid interstate competition in economic goods may damage the prosperity of the Nation as a whole, so Iowa’s attempt to deflect interstate truck traffic has been found to make the Nation’s highways as a whole more hazardous. That attempt should therefore be subject to “a virtually per se rule of invalidity.” Ibid.
This Court’s heightened deference to the judgments of state lawmakers in the field of safety, see ante, at 670, is largely attributable to a judicial disinclination to weigh the interests of safety against other societal interests, such as the economic interest in the free flow of commerce. Thus, “if safety justifications are not illusory, the Court will not second-*687guess legislative judgment about their importance in comparison with related burdens on interstate commerce.” Raymond Motor Transportation, Inc. v. Rice, supra, at 449 (Blackmun, J., concurring) (emphasis added). Here, the decision of Iowa’s lawmakers to promote Iowa’s safety and other interests at the direct expense of the safety and other interests of neighboring States merits no such deference. No special judicial acuity is demanded to perceive that this sort of parochial legislation violates the Commerce Clause. As Justice Cardozo has written, the Commerce Clause “was framed upon the theory that the peoples of the several states must sink or swim together, and that in the long run prosperity and salvation are in union and not division.” Baldwin v. G. A. F. Seelig, Inc., 294 U. S. 511, 523 (1935).
I therefore concur in the judgment.
Moreover, I would emphasize that in the field of safety — and perhaps in other fields where the decisions of state lawmakers are deserving of a heightened degree of deference — the role of the courts is not to balance asserted burdens against intended benefits as it is in other fields. Compare Raymond Motor Transportation, Inc. v. Rice, 434 U. S. 429, 449 (1978) (BlackmuN, J., concurring) (safety regulation), with Pike V. Bruce Church, Inc., 397 U. S. 137, 143 (1970) (regulation intended “to protect and enhance the reputation of growers within the State”). In the field of safety, once the court has established that the intended safety benefit is not illusory, insubstantial, or nonexistent, it must defer to the State’s lawmakers on the appropriate balance to be struck against other interests. I therefore disagree with my Brother Powell when he asserts that the degree of interference with interstate commerce may in the first instance be "weighed” against the State’s safety interests:
“Regulations designed [to promote the public health or safety] nevertheless may further the purpose so marginally, and interfere with commerce so substantially, as to be invalid under the Commerce Clause.” Ante, at 670 (emphasis added).
In the District Court and the Court of Appeals, Iowa’s attorneys forthrightly defended the regulation in part on the basis of the State’s interest in discouraging interstate truck traffic through Iowa. 475 F. Supp. 544, 550 (SD Iowa); 612 F. 2d 1064, 1069 (CA8 1979).
My Brother Rehnquist claims that the “argument” that a court should defer to the actual purposes of the lawmakers rather than to the post hoc justifications of counsel “has been consistently rejected by the Court in other contexts.” Post, at 702. Apparently, he has overlooked such cases as Allied Stores of Ohio, Inc. v. Bowers, 358 U. S. 522 (1959), where we described the rationale for our earlier decision in Wheeling Steel Corp. v. Glander, 337 U. S. 562 (1949):
“The statutes, on their face admittedly discriminatory against nonresidents, themselves declared their purpose. . . . Having themselves specifically declared their purpose, the Ohio statute left no room to conceive of any other purpose for their existence. And the declared purpose having been found arbitrarily discriminatory against nonresidents, the Court could hardly escape the conclusion . . . ,” 358 U. S., at 529-530.
And in Weinberger v. Wiesenfeld, 420 U. S. 636, 648, n. 16 (1975), we said:
“This Court need not . . . accept at face value assertions of legislative purposes, when an examination of the legislative scheme and its history demonstrates that the asserted purpose could not have been a goal of the legislation.” (Citing eases.)
And in Massachusetts Board of Retirement v. Murgia, 427 U. S. 307, 314 (1976), we stated that a classification challenged as being discriminatory will be upheld only if it “rationally furthers the purpose identified by the State.” See also Minnesota v. Clover Leaf Creamery Co., 449 U. S. 456, 463, n. 7 (1981); Califano v. Coldfarb, 430 U. S. 199, 212-213 (1977) (plurality opinion); Hughes v. Alexandria Scrap Corp., 426 U. S. 794, 813, n. 23 (1976); Johnson v. Robison, 415 U. S. 361, 381-382 (1974).
The extent to which we may rely upon post hoc justifications of counsel depends on the circumstances surrounding passage of the legislation. Where there is no evidence bearing on the actual purpose for a legislative classification, our analysis necessarily focuses on the suggestions of counsel, see Allied Stores of Ohio, Inc. v. Bowers, supra, at 528-529 (relied upon by the dissent, post, at 703-704, n. 13). Even then, “marginally more demanding scrutiny” is appropriate to “test the plausibility of the tendered *683purpose.” Schweiker v. Wilson, ante, at 245 (Powell, J., dissenting). But where the lawmakers’ purposes in enacting a statute are explicitly set forth, e. g., Minnesota v. Clover Leaf Creamery Co., supra, at 458-459; Johnson v. Robison, supra, at 376, or are clearly discernible from the legislative history, e. g., Hughes v. Alexandria Scrap Corp., supra, at 813, n. 23; McGinnis v. Royster, 410 U. S. 263, 274-277 (1973), this Court should not take — and, with the possible exception of United States Railroad Retirement Board v. Fritz, 449 U. S. 166 (1980), see id., at 187-193 (BreNnan, J., dissenting), has not taken — the extraordinary step of disregarding the actual purpose in favor of some “imaginary basis or purpose.” McGinnis v. Royster, supra, at 277. The principle of separation of powers requires, after all, that we defer to the elected lawmakers’ judgment as to the appropriate means to accomplish an end, not that we defer to the arguments of lawyers.
If, as here, the only purpose ever articulated by the State’s lawmakers for maintaining a regulation is illegitimate, I consider it contrary to precedent as well as to sound principles of constitutional adjudication for the courts to base their analysis on purposes never conceived by the lawmakers. This is especially true where, as the dissent’s strained analysis of the relative safety of 65-foot doubles to shorter trucks amply demonstrates, see post, at 694-696, the post hoc justifications are implausible as well as imaginary. I would emphasize that, although my Brother Powell’s plurality opinion does not give as much weight to the illegitimacy of Iowa’s actual purpose as I do, see Part III, infra, both that opinion and this concurrence have found the actual motivation of the Iowa lawmakers in maintaining the truck-length regulation highly relevant to, if not dispositive of, the case. See ante, at 677-678.
The veto message, printed at App. 626-631, is a complete statement of Governor Ray’s reasons for vetoing House Bill 671. App. 172 (deposition of Governor Ray).
It is not enough to conclude, as my Brother Powell does, that “the deference traditionally accorded a State’s safety judgment is not warranted.” Ante, at 678.