ALVIN B. RUBIN, Circuit Judge:
The Railroad Revitalization and Regulatory Reform Act of 1976 (the 4R Act)1 was designed to eliminate needless regulatory restraints on railroads and to prescribe rate-making practices that would both encourage effective competition and protect consumers.2 As one of its measures to achieve this, Congress limited the authority of the Interstate Commerce Commission to suspend a railroad rate on the basis that it is unjustly or unreasonably high. The Commission now has no jurisdiction to review any rate unless it finds that the rail carrier defending the rate can exclude effective competition for the transportation to which the rate applies. The carrier’s power to exclude such competition is called “market dominance.”
The Commission has adopted what it terms guidelines, but considers to be regulations, for deciding whether a carrier has market dominance. Under the guidelines, the Commission may consider evidence that other carriers or modes of transportation compete for the same movement of the product to which the rate applies. Evidence is also admissible that transportation of substitute products (product competition) or transportation of the same product from other places (geographic competition) provides less direct, but potentially equally effective, competition. When such evidence is considered, the number of rates immunized from regulation is increased.
Several organizations representing businesses and consumers that would be adversely affected by increases in various rail rates challenge the validity of the regulations on the ground that they exceed the Commission’s statutory authority and on the further ground that the regulations do not comply with the Commission’s statutory mandate to promulgate standards and procedures that facilitate market dominance determinations.
They also challenge other Commission decisions in the same regulations. The Commission has announced that user investments in rail-related facilities will not create a presumption of market dominance. Evidence of investments made in the future will not be admitted to show market dominance; evidence of past investments will be admitted but will create no presumption. In addition the Commission has abandoned the presumptions of market dominance formerly drawn from market share data and from cost/price ratios.
The Commission is joined in defending its actions by the Department of Justice and a number of intervenors including the Association of American Railroads and several railroad companies.
We conclude that the statute limits the definition of market dominance to transportation of the same product from the same origin to the same destination. The Commission’s definition of market dominance is, therefore, invalid. Because [383]*383the Commission must reconsider its definition, and must revise the standards and procedures adopted to implement it, any evaluation of those standards and procedures in their present form is unnecessary. We conclude, however, that the Commission did not exceed its statutory authority in adopting guidelines that describe the kind of evidence it will consider and the weight it will give such evidence.
I.
We preface the factual background with a review of established precepts. The administrative agency charged with executing a statute has primary responsibility for determining the scope of its authority.3 A reviewing court may not set aside the agency’s interpretation of the statute that authorizes it to act merely because the judges would have interpreted the statutory language differently.4 When, pursuant to congressional mandate, an agency adopts regulations to implement the statute it is charged with administering, the agency’s interpretation of the statute is entitled to even greater weight.5 Such regulations can be set aside only if the agency has exceeded its statutory authority or if its regulations so far depart from the statutory purpose that they can be stigmatized as “arbitrary or capricious,” or “an abuse of discretion, or “otherwise not m accordance with law.”6
These terms are necessarily imprecise for they do not express a formula. When Congress has chosen an administrative agency to fulfill both executive and quasi-legislative functions, a court reviewing the agency’s reading of the governing statute must take into account both the congressional grant of discretion to the agency and the agency’s expertise. Judicial review is not to be exercised with the zeal of a pedantic schoolmaster who grades papers for a single correct answer, but with the respect that is due responsible and experienced government officials themselves charged with duties of national importance.
Nonetheless, a reviewing court has the duty to determine whether an agency has transgressed the bounds fixed by Congress,7 else both judicial review and the constitutional stricture that ultimate legislative power not be delegated would both be meaningless. “[C]ourts are not bound to accept the administrative construction of a statute regardless of consequences.”8 While administrative agencies are expert in technical problems within their jurisdiction, they enjoy no special skill in statutory interpretation, an area in which courts are [384]*384the final authority.9 For these reasons a court reviewing an agency’s interpretation of its governing statute is not subject to the same constraints that apply when it reviews the procedures the agency adopts to fulfill its mandate.10
II.
For almost a century railroad rates, unlike prices charged in most sectors of the American economy, have been stringently regulated. Tariff rates could be changed only after the long delay occasioned by administrative litigation and review under the substantive standards of the Interstate Commerce Act. In 1976 Congress found that, due in no small part to excessive regulation, railroad facilities had deteriorated, return on investment was far below the cost of capital, and a succession of major railroad bankruptcies had occurred.11
At the same time, the central premise of maximum rate regulation, that the railroads had a transportation monopoly requiring control lest it become a monster, had itself become outdated. Although railroads dominated freight traffic in the United States when the Act was passed in 1887, railroads today carry only a third of intercity freight tonnage and compete in such traffic with motor carriers, barges, pipelines, airlines, and other forms of transportation.12 “[Tjhere are few significant commodities which are not practically susceptible to transportation by at least two competing modes of surface transportation[.]”13
Before 1976, all rail rates were required to be “just and reasonable.”14 The Commission had authority to regulate any rates that did not meet this standard. In adopting the 4R Act, Congress sought to restore financial stability to the railroad industry by reducing regulatory restraints on railroad pricing decisions. The Act permits the Commission to consider whether a rate is reasonable only if the Commission first determines that the rail carrier has “market dominance over the transportation to which [the] particular rate applies.”15 “Market dominance” was originally defined as “an absence of effective competition from other carriers or modes of transportation for the traffic or movement to which a rate applies.” 16 That language was subsequently revised, without substantive change, when the Interstate Commerce Act was recodified in 1978.17 In the 4R Act, Congress directed the Commission to establish, by rule, standards and procedures for making market dominance determinations.18 The [385]*385Commission was instructed to solicit and consider the recommendations of the Attorney General and the Federal Trade Commission in establishing its rules.19 The Act requires the Commission to make a jurisdictional market dominance determination within 90 days after the beginning of proceedings challenging the level of rail rates.20 The Staggers Act, adopted in 1980 21 adds a further limitation: the Commission shall find that the rail carrier establishing the challenged rate does not have market dominance unless the rate exceeds a stated revenue/variable cost ratio.22
III.
Soon after passage of the 4R Act, the Commission adopted Ex Parte No. 320, introducing rebuttable presumptions of market dominance. The Commission presumed that a carrier had market dominance if (1) the carrier handled 70 percent or more of the traffic of goods involved within the prior year (the “market share” presumption); (2) the rail rate yielded revenues exceeding 160 percent of the railroad’s variable cost of service (the “cost” presumption); or (3) the shipper had made a substantial investment in rail-related plant or equipment precluding the use of alternative transportation (the “substantial investment” presumption).23 The “cost” and “substantial investment” presumptions were rebuttable through the introduction of any relevant evidence. The “market share” presumption could be rebutted only by evidence of direct carrier competition for the specific movement.
The Department of Justice, the Department of Transportation, and the railroads had urged that the definition of the relevant market should not be so confined but should extend more broadly to include product, geographic, and potential competition.24 The Commission gave two reasons for rejecting that position. Such evidence would create complex antitrust-type litigation that could not be resolved within the 90-day statutory time limits25 for making market dominance decisions. In addition, the Commission concluded that the statutory definition of market dominance precluded consideration of such competition.26 The Commission also pointed out that, as experience was gained in the implementation of the concept of market dominance, refinements and modifications might be required. It would hold the proceedings open to permit reevaluation of the various standards and procedures, based on actual experience.27
The Commission’s regulations were upheld on judicial review, with the exception of a limited remand for purposes of clarifying the cost presumption. Atchison, T. & S.F. Ry. v. ICC, 580 F.2d 623 (D.C.Cir.1978). In that case, as here, a major issue before the Court concerned the Commission’s treatment of geographic and product competition. The Department of Justice argued that the Commission erred in not permitting such evidence to be considered even to rebut the regulatory presumptions. In upholding the Commission’s decision to exclude such evidence, the court found “sufficient basis in the statutory language and purpose to merit our deferral to the Commission’s view.”28
Commenting generally on the respective role of the Commission and the courts in implementing the market dominance regulations, the District of Columbia court stated:
[386]*386Overall, the Court’s role is one of deference and deferral. The Commission will be in a position to evaluate the regulations more fully in light of experience. That is an important feature of the administrative process. The courts remain open if the Commission is slothful or unwilling to undertake appropriate reconsideration and fine tuning in the light of experience.
580 F.2d at 623 (citations omitted).
In response to the court’s remand for clarification of the cost presumption, the Commission issued a supplementary opinion.29 In that opinion, the Commission noted that it was undertaking a further analysis of its market dominance definition that would, for example, “reveal how to treat evidence of geographic competition.”30 The Commission emphasized that it would look at all pertinent evidence and noted that, although certain kinds of competition are not relevant to computing market share, carriers could introduce evidence of geographic and product, as well as other, competition, to establish that effective competition exists.31
Ex Parte 320 (Sub-No. 1)
After several years of experience with the presumptions,32 and after completion of several studies,33 the Commission proposed in Ex Parte No. 320 (Sub-No. 1)34 removal of all but the cost presumption. In lieu of the eliminated presumptions, the Commission proposed that parties present any relevant evidence, including evidence of geo[387]*387graphic or product competition. While the Commission was considering comments in that proceeding, however, Congress enacted new legislation which required modification of that proposal.
The Staggers Act Amendment
In 1980, Congress amended the market dominance section by restricting the situations in which the Commission might find market dominance. Section 202 of the Staggers Act directs the Commission to make a finding that the carrier lacks market dominance if the carrier shows that the challenged rate would yield revenues less than a given percent of variable cost.35 Contrary to the Commission’s previous cost presumption, the amended statute provides that a finding that a rate is equal to or greater than the statutory variable cost percentage “does not establish a presumption” of market dominance.
The Challenged Decision:
1. Elimination of the Market Dominance Presumptions
Following passage of the Staggers Act, the Commission adopted the challenged regulations. The market dominance presumptions have been eliminated and replaced with broader and more flexible guidelines. The primary reason for adopting the new approach is the Commission’s conclusion that the presumptions do not necessarily reflect railroad market power and, therefore, yield inaccurate market dominance determinations.36 The cost, market share, and substantial investment presumptions are found to be poor indicators of market dominance in the widely varying fact situations to which they must be applied. The new guidelines focus market dominance evidence on the transportation alternatives, if any, available to a shipper, and the feasibility of using each.37
2. Cost Presumption
With respect specifically to elimination of the cost presumption, the Commission notes that the former presumption (i.e., a rate exceeding 160 percent of variable cost establishes market dominance) is inconsistent with the Staggers Act.38 That act removes from Commission jurisdiction rates below a threshold figure set at 160 percent for 1980 and increasing annually to 180 percent in 1984. The Commission rejects suggestions that it adopt a higher ratio as a presumption of market dominance, noting that although price/cost ratios provide a convenient benchmark for deciding whether to look for market dominance (the role assigned by the jurisdictional provision of the Staggers Act), there are a number of reasons why a high price/cost ratio may not be indicative of true market power.
[388]*388Ratios do not, for instance, tell us about the degree of market power possessed by the railroad, since they do not tell us whether a proposed rate will actually move traffic over an extended period of time. If the rate is high, shippers may find alternatives more attractive, forcing the rate back down again. Some may accept the high rate because of a preference for the carrier or because of a premium service associated with it. There are any number of reasons why a high price/cost ratio may not be indicative of true market power on the part of the railroad. Reliance on such ratios will, therefore, not only be misleading, but will preclude more relevant information from being introduced.39
As an additional reason for eliminating the presumption, the Commission recites the difficulties in determining the variable costs of a rail movement.
Such costs depend on numerous factors that vary considerably from one movement to another. Since the simplicity of the cost test requires that a standard costing methodology be used, there is no way of avoiding the distorting inaccuracies of such a test. Many rates falling above a designated revenue-to-variable cost ratio would, on the basis of more accurate cost estimates, in fact be below it. Since more accurate cost information can, and very likely will, be presented as evidence, we think a price/cost presumption based on imperfect, though standard, cost estimates offers very little help.40
3. Market Share Presumption
The Commission’s reason for eliminating the market share presumption is the difficulty it has found in defining the scope of the market.41 The basic problem, the Commission points out, is that the factors determining the appropriate scope vary widely in individual situations. Moreover, the Commission asserts that even an accurate calculation of a carrier’s share of a particular market reflects only the transportation alternatives selected in the past, as opposed to alternative sources currently available. The competitive implications of any given market share percentage, therefore, vary widely from case to case.
4. Substantial Investment Presumption
The Commission states that the substantial investment presumption was designed to protect shippers tied to a particular carrier by substantial rail-related investment.42 Its decision to eliminate the presumption is based on two considerations. First, the Commission has experienced difficulty in identifying pertinent elements of a shipper’s investment in plant and equipment and in determining whether that investment makes use of another carrier or method impossible or impractical. Explaining its decision to eliminate the presumption, the Commission asserts that the “amount of rail-related investment is, by itself, a poor index of captivity.”43 As an example, it cites the purchase of coal cars, which may represent a new and substantial rail-related investment, yet not tie shippers to a particular railroad.
Second, the ability of shippers and carriers to enter into rail service contracts has reduced the importance of substantial investment in market dominance proceedings.44 Shippers can now protect them[389]*389selves, the Commission states, by negotiating with various carriers. When sufficient competition exists, carriers will have the incentive to offer contracts with reasonable terms.45
The Commission adopts substantially the same policy with respect to long-term coal supply contracts, in which shippers tie themselves to particular carriers by negotiating long-term contracts with coal sources served by only one railroad46 The Commission has decided to place considerably less weight on the existence of long-term coal supply contracts executed after passage of the Staggers Act. It is concerned that shippers will not take advantage of opportunities to negotiate with carriers, but will rely instead on the old presumptions. In effect, the Commission has announced that it will not allow shippers to ensure its continuing jurisdiction over their rail transportation after October 1980 by tying themselves to a single carrier with substantial rail-related investments or long-term supply contracts.
5. Geographic and Product Competition
The Commission’s guidelines state that four major forms of competition affect rail transportation: (1) intramodal competition, i.e., competition by railroads with railroads; (2) intermodal competition, i.e., competition with railroads by other forms of transportation; (3) geographic competition; and (4) product competition.47 Under each of these headings, the guidelines list representative factors relevant to the existence of competition.48 The guidelines indicate that other forms of competition may also be important in individual circumstances and that the Commission is prepared to consider such evidence on a case-by-case basis.49
Because of its prior position on the limit-' ed applicability of evidence concerning geographic and product competition, the Commission discusses at length its decision to include such evidence.50 First, the Commission addresses its original interpretation of the statutory language and concludes that its reading was unnecessarily restrictive in focusing only on direct carrier competition and ignoring the indirect competitive impact that geographic or product competition may have.51 The Commission finds nothing in the 4R Act or the Staggers Act that precludes revising its original restrictive interpretation.52
In response to arguments that consideration of geographic and product competition is not possible within the time allotted, the Commission states that determining the presence or absence of effective geographic competition under the new guidelines should be manageable in most cases. Finally, the Commission addresses its previous concern that such evidence can not establish the existence of truly effective competition because geographic or product competition does not provide an immediate restraint on prices.53 Although those forms of competition are less effective in restraining prices than a possible immediate loss of traffic, the Commission now recognizes the deterrent effect of a future loss of revenues on short-term market abuse.
In order to clarify the procedures for introducing evidence of market dominance, the Commission first notes that the railroad has the statutory obligation to present revenue/variable cost evidence if it seeks to [390]*390demonstrate that the rate is below the stat- ' utory jurisdictional threshold. The railroad must also introduce evidence of product or geographic competition by identifying such competition. Once the railroad has made this identification, the party opposing the rate has the burden of proving that the product or geographic competition identified by the railroad is not effective.
Thus, the Commission’s new guidelines reflect, in part, the positions urged original- • ly by the agencies with statutory responsibility for advising it. After five years of experience with a more cautious approach, the Commission has found merit in the FTC’s original criticism that use of presumptions as “shortcuts” obscures other considerations of relevance to the market dominance determination.54
IV.
The petitioners challenge the Commission’s position that the statute gives it authority either to consider or not to consider evidence of geographic and product competition in determining market dominance. To evaluate this position, we first return to the statute. It refers to “effective competition ... for the transportation to which a rate applies,”55 not to competition with that transportation.
The statutory language must be read in the light of the manner in which the market dominance issue arises. When a shipper challenges a specific rate to transport a particular product from one place to another, the Commission has jurisdiction to review the reasonableness of the rate only because the rail carrier setting the rate has market dominance over that particular movement. Competition for a movement of coal by rail from a coal deposit in Wyoming to a utility company in Texas, for example, is created only if some other carrier is willing to move that product from the same origin to the same destination. The availability of coal moving by barge from a mine in Kentucky to the Texas utility (geographic competition) creates competition with the Wyoming-Texas movement but not competition for that movement. Similarly, suppliers of natural gas seeking to deliver it by pipeline to the utility may compete economically with the sellers of coal to supply an alternate energy source, but neither the natural gas producers nor the pipeline delivering their product compete for the movement to which the challenged rate applies.
The congressional history does not indicate that the Commission was meant. to consider geographic and product competition in determining whether a carrier has market dominance. The Senate Conference Report on the 4R Act merely states:
While the absence of effective competition test is not intended to strictly conform with the standards of the antitrust laws, it is intended that when the Commission administers the test it will recognize the absence of forces which normally govern competitive markets.” (Emphasis added.)
1976 U.S.Code Cong. & Ad. News 148, 163. Far from requiring the Commission to recognize the presence of product-geographic competition, this language mandates only that the Commission recognize the absence of competitive forces in the railroad industry.
The Staggers Act demonstrates that Congress knew the economic significance of product-geographic competition and referred specifically to those factors when it thought them appropriate. Section 205(a)(1) of the act requires the Commission to initiate a proceeding to determine whether, and to what extent, “product competition” (defined by the statute to include both what we have called product and geographic competition) should be considered in determining the reasonableness of rates. The instruction is not set forth in the U.S.Code [391]*391but is contained in the historical note following 49 U.S.C.A. § 10701a (West Special Pamphlet 1982). Section 205(a) expressly recites that this shall not be construed as altering the meaning of the term “market dominance” as defined in 49 U.S.C. § 10709(a). Pub.L. No. ' 96-448, § 205(a)(3)(B), 94 Stat. 1905 (1980).56
The Commission’s interpretation of the statute would read “effective competition ... for the transportation to which a rate applies” to mean “effective competition with the transportation.” There may be sound economic or administrative arguments for giving the Commission such free rein, but Congress simply did not do so. The Commission’s studies showing that direct competition for a particular movement is not a sufficient economic test may indicate how much wiser Congress would have been had it allowed the Commission more latitude. Those studies and the Commission’s recommendations may, indeed, persuade Congress to alter the statute. But the post-enactment economic studies tell us nothing about what the statute means.
While the Commission’s original interpretation of the statute as excluding consideration of product-geographic competition is not decisive, it is not to be ignored. In deferring to the Commission’s original interpretation, the District of Columbia Circuit commented “the construction may appear to some to attribute excessive significance to a terse statutory term.” Santa Fe, supra, 580 F.2d at 634. The tentativeness of this endorsement does not constitute approval of a completely different reading. The statute is certainly brief: the entire definition of market dominance is set forth in twenty words. But brevity does not necessarily imply ambiguity. When a statute is clear, we should read it to mean what it says, however succinctly.57
We do not imply that, once the Commission makes a statutory interpretation, it is bound by a sort of agency stare decisis. Based on what the Commission learns from experience, new information, or new ideas, it may indeed alter its interpretations, overturn its administrative rulings, and adopt new practices. American Trucking Associations, Inc. v. Atchison T. & S.F. By. Co., 387 U.S. 397, 416, 87 S.Ct. 1608, 1618, 18 L.Ed.2d 847, 860 (1967). Error is not to be perpetuated simply because it has been once made, and wisdom is not to be rejected merely because it comes late.
Here, however, there is no evidence the Commission has learned one iota more about the meaning of the statute or the direction of Congress than it knew five years ago. The Commission has merely altered its reading of what Congress directed a half-decade earlier because it considers that the new interpretation would produce more desirable results. This after-acquired wisdom cannot alter the statutory language and allow the Commission unfettered authority to define effective competition as it will.
Y.
The petitioners also contend that the Commission’s guidelines contravene the congressional directive in the 4R Act, that it
establish, by rule, standards and procedures for determining ... whether and when a carrier possesses market dominance .... Such rules shall be designed [392]*392to provide a practical determination without administrative delay.58
Necessarily our determination will require the Commission to reconsider its standards and procedures. However, because some of the questions raised and fully briefed in this appeal must be considered by the Commission and may, therefore, recur, we now consider them.
The Commission, it is argued, has promulgated “guidelines” rather than “rules,” and these do not establish standards or procedures that will permit a practical determination. It is of no moment that, for reasons it has failed to articulate, the Commission has clad its promulgation in the cloak of guidelines rather than rules. The Administrative Procedure Act59 defines a “rule” as “the whole or a part of an agency statement of general or particular applicability and future effect designed to implement, interpret, or prescribe law or policy or describing the organization, procedure, or practice requirements of an agency ... . ” The Commission followed the notice and comment procedures for rulemaking prescribed by the Administrative Procedure Act. 5 U.S.C. § 553 (1976). None of the petitioners challenges the fairness of the procedure or the Commission’s compliance with the rulemaking procedures. Although the Commission’s decision embodies statements of policy and interpretations, the Commission also clearly intended to announce a change in the method it would follow in making the market dominance determination, a determination affecting substantive rights.60
In American Trucking Associations, Inc. v. ICC, 659 F.2d 452 (5th Cir.1981), we discussed the differences between guidelines and rules and the criteria to be applied in determining whether, in a given situation, the distinction is merely semantic. As we there held, the status of guidelines as "rules” is determined by their binding character.61 The Commission’s guidelines describe the “obligations” of railroads and shippers (365 I.C.C. 131-32); direct shippers to “use these guidelines for preparation of rebuttal evidence” (id. at 132); and state that the Commission “will” give more or less weight to certain evidence (id. at 131) and “will” consider product and geographic competition evidence (id. at 135). In short, as in American Trucking, there are “sinews of command beneath the velvet words of the ... guidelines.” 659 F.2d at 463.62 The guidelines here involved are indeed rules by these criteria and the Commission’s choice of nomenclature is without legal significance. Thus, the Commission’s guidelines conform to the 4R Act’s “rule” requirement.
Whether or not the Commission can feasibly make market dominance determinations within ninety days based on its guidelines is a matter for it to determine, at least in the first instance. Whatever the complexity of the issues before it in determining market dominance, the Commission is the best arbiter of its ability adequately to consider those factors within the time allowed. Vermont Yankee Nuclear Power Corp. v. Natural Resources Defense Council, Inc., 435 U.S. 519, 524-25, 98 S.Ct. 1197, [393]*3931202, 55 L.Ed.2d 460, 467-68 (1978). We decline to tell the Commission in advance of any experience that its procedures prevent it from reaching a decision within the time allowed it by statute.
Petitioners also attack the sufficiency of the guidelines to constitute “standards,” as Congress commanded. They argue that the guidelines lack the definiteness and predictability of standards and, therefore, do not allow the Commission and parties to measure in a practical way whether there is effective competition in a particular situation. Because our decision requires the Commission to develop new rules that may be more or less explicit than the present ones, we need not now decide how precise administrative declarations of policy must be to constitute standards. For the same reasons, we decline to rule on the argument that the guidelines do not lend themselves to the “practical determination” of market dominance exacted by Congress.
VI.
In addition to its decisions concerning product-geographic competition, the Commission’s guidelines eliminate the presumption of market dominance flowing from substantial shipper investment in rail-related plant or equipment. Instead, the 1980 guidelines announce that evidence of substantial investment made prior to October 1, 1980 (the date of passage of''the Staggers Act) can be introduced as “an evidentiary tool.”63 Investments made after that date will not be considered. The petitioners contend that this arbitrarily excludes evidence of post-October 1, 1980 investments, and is unreasonable even as to investments made before that date.
As to pre-October 1, 1980 investments, the Commission has merely removed the presumption, rebuttable even in its original formulation, that a carrier has market dominance when affected shippers or consignees “have made a substantial investment in rail-related equipment which prevents or makes impractical the use of another carrier or mode.”64 The Commission’s reasons for doing so have already been noted. As to post-October 1, 1980, investments, the Commission reasons that “contracts afford adequate protection to shippers with substantial rail-related investment.”65 The Commission also deals with the argument “that such contracts encompass too short a period of time to cover the life of most rail-related investments.”
For example, the Chemical Manufacturers Association indicated that, because of many uncertainties, railroads are generally reluctant to engage in long-term contracts and that few of these contracts exceed 5 years. We do not deny that the protection afforded by a contract becomes less effective to the extent that the life of that contract is less than the life of the rail-related investment. However, we also emphasize that such problems are not beyond the control of the shipper. A long-term contract that protects the shipper for the life of the investment should be available from a railroad who is encouraging that investment to be made, given the business that it means for the railroad. Furthermore, the historical failure of shippers to negotiate long-term contracts may, in part, have been due to the protection provided by the substantial investment presumption.
365 I.C.C. at 125.
Contrary to petitioners’ assertion, the Commission is not forcing shippers who make substantial investments in rail-related equipment after the Staggers Act either to enter into a rail transportation contract or to forego maximum rate protection under the Interstate Commerce Act. Its decision encourages shippers to use whatever bargaining position they have to negotiate with carriers for transportation before making substantial investments. The Commission has professed a readiness to recognize the absence of transportation alternatives in particular situations and to afford [394]*394those shippers adequate protection. This policy does not, on the face of it, seem arbitrary in an era where railroads and shippers are free to contract under the Interstate Commerce Act. To the extent that the Commission’s guidelines encourage shippers with transportation alternatives to negotiate for transportation contracts, the guidelines are consistent with Congressional policy.66
Ultimate correctness, like truth, is difficult to ascertain. The Commission’s reasons are plausible. Petitioners seek to demonstrate the untenability of the Commission’s decision by argument rather than evidence. It is not our role to reassess the decision de novo, but only to reject it if it is arbitrary or capricious. Nothing in the record and nothing of which we might take judicial notice enables us to stigmatize that decision as willfully adopted, without support or reasonable explanation.
VII.
Finally, the petitioners protest elimination of the cost and market share presumptions of market dominance. Although the guidelines eliminate the presumptions, they do not arbitrarily exclude pertinent evidence. The Commission encourages the submission of evidence of “more accurate costing information which may include price-cost ratios and lead to more appropriate market determinations.”67 It also promises to consider evidence of current market shares and market share trends. The Commission’s half-decade of experience with the presumptions equips it to decide whether the presumptions have accomplished their purpose. We lack the knowledge to evaluate the Commission’s conclusions and the warrant to substitute our judgment. Certainly we cannot brand the conclusions arbitrary or capricious.
CONCLUSION
For these reasons, we hold that the Commission’s rule allowing evidence of product and geographic competition to be considered in deciding whether a carrier has market dominance violates the limits contained in the statutory definition and is invalid. We express no opinion whether its “guidelines” are sufficient to be “standards” because they must be recast after new rules concerning market dominance are framed. We uphold the Commission’s rules eliminating cost, market share, and substantial investment presumptions.
The petitioners invite us to provide interim rules for the Commission by reinstating its former guidelines. This we decline to do. The Commission itself is best equipped to decide how it will formulate regulations consistent with this opinion and what procedures it will adopt to handle its task in the interim.
The case is REMANDED for further proceedings consistent with this opinion.