[439]*439JON P. WILCOX, J.
¶ 1. This case is before the court on certification from the court of appeals following an order of the Circuit Court for Kenosha County, David M. Bastianelli, Judge, which dismissed the appellants' (collectively referred to as Catholic Mutual) motion for summary judgment upon Catholic Mutual's cross-claim for contribution against the respondent American Family Insurance Company (American Family). Catholic Mutual appealed from the circuit court's final order.
¶ 2. As we interpret this case, there is one issue presented for our determination: whether a common law action for contribution may be brought against persons who violate Wis. Stat. § 347.48(2m)(c) (1989-90),1 as controlled by § 347.48(2m)(g), by operating a motor vehicle without reasonably believing that each passenger between 4 and 15 years of age, and seated at a designated seating position in the vehicle, is properly restrained with a seat belt. The court of appeals presented the following two issues on certification: (1) whether the passive negligence of a non-intentional negligent tortfeasor creates a common liability with a causally negligent tortfeasor supporting a claim of contribution for enhanced injuries attributable to the passive negligence; and (2) whether there can be contribution in an enhanced injury case from a party whose passive negligence was a substantial cause of the enhanced injuries.
¶ 3. We need not address these issues as they are certified to this court, since we conclude that the legislature has expressed its intent that a claim for contribution may not be sustained in cases involving negligence for failure to restrain another with a seat [440]*440belt, as controlled by Wis. Stat. § 347.48(2m)(g). Accordingly, we affirm the order of the circuit court which dismissed Catholic Mutual's motion for summary judgment.
¶ 4. In the proceedings below, the parties stipulated to the relevant facts in this matter. On September 11, 1991, the plaintiff Robin Gaertner (Gaertner) picked up 11 year-old Justin Koldeway (Koldeway) from school at the request of Koldeway's mother and drove him to a doctor's appointment. Following the appointment, Gaertner, a friend of Koldeway's mother, began driving to her home with Koldeway seated in the rear seat of the car. On the way home, Gaertner was involved in an accident with an automobile driven and owned by the defendant Gertruda Holcka (Holcka). The accident was caused solely by Holcka's negligence.
¶ 5. At the time of the accident, Koldeway was not wearing an available rear shoulder harness seat belt installed for his seat. Gaertner operated her automobile without reasonably believing either prior to or at the time of the accident that Koldeway was wearing the seat belt.
¶ 6. Koldeway sustained serious and permanent injuries in the accident, amounting to $588,235.29 in damages. As a full and final settlement of Koldeway's claims arising out of the accident, Catholic Mutual, Holcka's insurer, paid $500,000 to Koldeway. At the same time, Catholic Mutual preserved its right to seek contribution from Gaertner's insurer, American Family, which had issued a $100,000 insurance policy to Gaertner that was in effect on the date of the accident.
¶ 7. The $500,000 settlement appears to represent 85% of the total damages suffered by Koldeway, as reduced by the parties' apparent inter[441]*441pretation of Wis. Stat. § 347.48(2na)(g). The relevant provisions of this statute provide:
Safety belts and child safety restraint systems. ...
(2m) Required Use. (a) In this subsection, "properly restrained" ‘means wearing a safety belt approved by the department under sub. (2) and fastened in a manner prescribed by the manufacturer of the safety belt which permits the safety belt to act as a body restraint.
(c) If a motor vehicle is required to be equipped with safety belts in this state, no person may operate that motor vehicle unless he or she reasonably believes that each passenger who is at least 4 years old and not more than 15 years old and who is seated at a designated seating position in the front seat required under 49 CFR 571 to have a safety belt installed or at a designated seating position in the seats, other than the front seats, for which a shoulder harness has been installed is properly restrained.
(d) If a motor vehicle is required to be equipped with safety belts in this state, no person who is at least 4 years old and who is seated at a designated seating position in the front seat required under 49 CFR 571 to have a safety belt installed or at a designated seating position in the seats, other than the front seats, for which a shoulder harness has been installed may be a passenger in that motor vehicle unless the person is properly restrained.
(g) Evidence of compliance or failure to comply with par. (b), (c) or (d) is admissible in any civil action for personal injuries or property damage [442]*442resulting from the use or operation of a motor vehicle. Notwithstanding s. 895.045, with respect to injuries or damages determined to have been caused by a failure to comply with par. (b), (c) or (d), such a failure shall not reduce the recovery for those injuries or damages by more than 15%. This paragraph does not affect the determination of causal negligence in the action.2
¶ 8. Wisconsin Stat. § 895.045, as cross-referenced in § 347.48(2m)(g), is Wisconsin's contributory-negligence statute. It provided as follows:
Contributory negligence. Contributory negligence shall not bar recovery in an action by any person or his legal representative to recover damages for negligence resulting in death or in injury to person or property, if such negligence was not greater than the negligence of the person against [443]*443whom recovery is sought, but any damages allowed shall be diminished in the proportion to the amount of negligence attributable to the person recovering.
¶ 9. Dr. Joel Myklebust, a qualified biomechanical engineer expert witness, opined that Koldeway's damages would have been reduced substantially had Koldeway been wearing a seat belt at the time of the accident. According to the expert, 75% of Koldeway's injuries were caused by the failure to wear a seat belt and 25% were caused by the accident.
¶ 10. Of the 75% of injuries caused by failure to wear a seat belt, 70% of Koldeway's incremental injuries were caused by Gaertner's operation of her automobile without reasonably believing or ensuring that Koldeway was wearing a seat belt, and 30% were caused by Koldeway's own failure to wear a seat belt.
¶ 11. Gaertner subsequently brought suit against Catholic Mutual to recover damages for injuries which she sustained in the accident. In turn, Catholic Mutual filed a cross-claim against American Family seeking contribution for a portion of the $500,000 which it had paid to Koldeway. Specifically, Catholic Mutual calculated its contribution damages as follows: $500,000.00 x 0.75 x 0.70 = $308,823.53.3
[444]*444¶ 12. Stated differently, Catholic Mutual multiplied the total insurance settlement paid to Koldeway by the percentage of Koldeway's injuries that were caused by the failure to wear a seat belt alone. This product was in turn multiplied by the percentage of Koldeway's injuries, enhanced by failure to wear a seat belt, that were caused by Gaertner's operation of her automobile without reasonably believing that Koldeway was wearing a seat belt at the time of the accident.
¶ 13. According to Catholic Mutual, the net amount represents the percentage of injuries caused by Gaertner's failure to ensure that Koldeway was wearing a seat belt. Therefore, Catholic Mutual argues that it was entitled to judgment for the entire $100,000 American Family policy.
¶ 14. Following a stipulated dismissal of Gaertner's claims against the defendants, Catholic Mutual filed a motion for summary judgment, arguing that the three elements of a contribution claim were satisfied in this case. "The three prerequisites to a contribution claim are: 1. Both parties must be joint negligent wrongdoers; 2. they must have common liability because of such negligence to the same person; [and] 3. one such party must have borne an unequal proportion of the common burden." General Accident Ins. Co. v. Schoendorf & Sorgi, 202 Wis. 2d 98, 103, 549 N.W.2d 429 (1996) (quoting Farmers Mutual Auto. Ins. Co. v. Milwaukee Auto. Ins. Co., 8 Wis. 2d 512, 515, 99 N.W.2d 746 (1959)).
¶ 15. Specifically, Catholic Mutual argued that both itself and Gaertner were jointly liable for Koldeway's enhanced injuries because their independent torts concurred in time. Because the joint negligence of the parties caused the enhanced injuries, [445]*445Catholic Mutual asserted that the parties had common liability as well. Finally, Catholic Mutual argued that it had borne an unequal proportion of the common liability since it paid for all of Koldeway's damages arising out of the accident.
¶ 16. In response, American Family argued that neither the legislative history of Wis. Stat. § 347.48, nor the common law in this state has ever created a right of contribution for negligent tortfeasors in seat belt situations. To the contrary, American Family asserted that the legislature's choice to limit the reduction of damages recoverable by the injured party to 15% evidenced an intent to prevent the tortfeasor responsible for the accident from receiving a "windfall" other than the 15% provided by statute.
¶ 17. The circuit court concluded that since the accident was caused solely by Holcka's conduct, there was no common liability in this case. Because there was no common liability for the accident, Catholic Mutual was not entitled to contribution as a matter of law. Accordingly, on July 3, 1996, the circuit court denied the motion for summary judgment, and entered judgment in favor of American Family. Catholic Mutual appealed, and the court of appeals certified the case to this court pursuant to Wis. Stat. § (Ride) 809.61 (1995-96).
I.
¶ 18. The issue presented is whether a common law action for contribution may be brought against persons who violate Wis. Stat. § 347.48(2m)(c), as controlled by § 347.48(2m)(g), by operating a motor vehicle without reasonably believing that each passenger between 4 and 15 years of age, and seated at a [446]*446designated seating position, is properly restrained. Whether Catholic Mutual's motion for summary judgment should have been granted on this issue is a question of law that the appellate courts may review without deference to the circuit court's analysis. See Santiago v. Ware, 205 Wis. 2d 295, 323, 556 N.W.2d 356 (Ct. App. 1996). Although we follow substantially the same methodology employed by the circuit court in analyzing a motion for summary judgment, the facts of this case are undisputed and, as such, we need not engage in the step-by-step analysis which that methodology requires. See id.
¶ 19. Before addressing the issue, we first trace the history of common law and legislation involving seat belt negligence in Wisconsin. Since 1967, Wisconsin has recognized that the failure to utilize an available seat belt could be a possible defense to a personal injury claim. See Bentzler v. Braun, 34 Wis. 2d 362, 385, 149 N.W.2d 626 (1967). Although the seat belt law did not then require use of a seat belt, we concluded that "there is a duty, based on the common-law standard of ordinary care, to use available seat belts independent of any statutory mandate." Id.
¶ 20. Use of the "seat belt defense" was later discussed and clarified in Foley v. City of West Allis, 113 Wis. 2d 475, 335 N.W.2d 824 (1983). In Foley, we addressed the possible ramifications of the successful seat belt defense on liability or damages, "since the defense was not proved in Bentzler.” Id. at 484. In characterizing the defense, we stated that: "[s]ince failure to wear seat belts generally causes incremental injuries, damage for these incremental injuries can be treated separately for purposes of calculating recoverable damages." Id. at 485. The incremental damages caused by seat belt negligence could be distinguished [447]*447from those produced by passive negligence, where the damages are identical to the injuries caused by the active negligence in the same accident and are difficult to separate for purposes of calculating recoverable damages. See id.
¶ 21. As a result, we determined that seat belt negligence was not synonymous with ordinary passive negligence, but rather was more akin to an accident involving two incidents: the first incident being the actual automobile collision, and the second occurring when the occupant of the vehicle hits the vehicle's interior. See id. at 484-85. These differences led us to conclude that "a fair and administrable procedure.. .is to calculate a plaintiffs provable damages by the usual rules of negligence without regard to the seat belt defense and then take into account the seat belt defense by decreasing the recoverable damages by the percentage of the plaintiffs causal seat belt negligence." Id. at 486-87.
¶ 22. Stated as a mathematical formula, we adopted the following method for calculating damages when a successful seat belt defense is employed:
(1) Determine the causal negligence of each party as to the collision of the two cars (Table 1); (2) apply comparitive negligence principles to eliminate from liability a defendant whose negligence causing the collision is less them the contributory negligence of a plaintiff causing the collision (Table 1); (3) using the trier of fact's calculation of the damages, reduce the amount of each plaintiffs damages from the liable defendant by the percentage of negligence attributed to the plaintiff for causing the collision (Table 1); (4) determine whether the plaintiffs failure to use an available seat belt was negligence and a cause of injury, and if so what percentage of the [448]*448total negligence causing the injury was due to the failure to wear the seat belt (Tables 2 and 3); (5) reduce the plaintiffs damages calculated in step (3) by the percentage of negligence attributed to the plaintiff under step (4) for failure to wear an available seat belt for causing the injury.
Id. at 490.
¶ 23. Steps four and five of this test were subsequently altered by the legislature in 1987. See 1987 Wisconsin Act 132; see also Wis. Stat. § 347.48. Legislative history indicates that the legislature intended to limit Foley's effect on the reduction of a plaintiffs recovery for damages that were caused solely by failure to wear a seat belt:
The provision on personal injury actions revises Wisconsin common law, as formulated by the Wisconsin Supreme Court in Foley v. City of West Allis, 113 Wis. 2d 475, 335 N.W.2d 824 (1983). Under Foley, if negligent failure to wear a safety belt is a cause of the injured person's injuries, the injured person's recoverable damages are to be reduced by the percentage of damages caused by failure to wear a safety belt. Under Act 132, recoverable damages may not be reduced by more than 15%, regardless of the percentage of damages caused by failure to wear a safety belt.
Information Memorandum 87-8 at 8, Wisconsin Legislative Council Staff, January 8, 1988. See also Legislative Reference Bureau Memorandum, October 27,1987.
¶ 24. Thus, the legislature eliminated the possibility left open by Foley that seat belt negligence causing incremental injuries could disrupt the distribution of financial responsibility to any great degree. Prior to the legislature's action in 1987, a plaintiffs [449]*449recoverable damages, and, conversely, a defendant or defendants' overall financial responsibility, could have been significantly reduced in situations where the failure to wear a seat belt caused the majority of plaintiff s injuries. Since the amendment of Wis. Stat. § 347.48, the reduction in plaintiffs recoverable damages, and the corresponding "benefit" received by defendants, is statutorily limited to 15% of the injuries caused by failure to wear a seat belt.
II.
¶ 25. It is against this background that we make our decision regarding actions for contribution by a defendant's insurer against a plaintiff who is responsible for failing to ensure that an injured minor was properly restrained in a seat belt at the time of an automobile accident. Several considerations are relevant to our decision.
A.
¶ 26. First, it is important to recognize that we have never interpreted the seat belt defense to provide an affirmative cause of action for contribution. As we stated in Foley, seat belt negligence is not to be included in the same class as active or passive negligence. See Foley, 113 Wis. 2d at 484-86. Instead, the incremental injuries that are caused by the failure to wear a seat belt "can be treated separately for purposes of calculating recoverable damages." Id. at 485. This distinction "borrows from the apportionment technique used in two traditional tort doctrines: avoidable consequences and mitigation of damages." Id. at 487.
¶ 27. Thus, it is clear that we have previously interpreted the seat belt defense to be just that: a [450]*450defense. When a party fails to wear a seat belt, he or she has presumptively failed to mitigate his or her damages. Defendants may assert plaintiffs failure to "buckle up" in defending against a cause of action for personal injury and negligence. As we made clear in Foley, the defense may not be used to affect the causal negligence in a personal injury action.4 Instead of being viewed as ordinary negligence that could be used in an affirmative action to recover damages, then, the common law seat belt defense was seen as a useful tool to ensure "that the defendant is not held liable for incremental injuries the plaintiff could and should have prevented by wearing an available seat belt." Id. at 489.
B.
¶ 28. By amending Wis. Stat. § 347.48, the legislature explicitly adopted our interpretation of the seat belt defense. Significantly, the legislature sought to preserve Foley's attempt to prevent defendants from attaining a windfall by indicating that "[t]his paragraph does not affect the determination of causal negligence in the action." See Wis. Stat. § 347.48(2m)(g). As it is relevant to this opinion, the legislature modified the common law in only two ways: (1) to limit to 15% the potential reduction in plaintiffs' recoverable damages; and (2) to establish a duty on [451]*451behalf of the driver to properly restrain minor passengers.5 We will address these changes in turn.
¶ 29. The change effected by subsection (2m)(g) (15% maximum reduction in plaintiffs recoverable damages) does not, on its face, illustrate any intent to create an affirmative cause of action for contribution. To the contrary, as explained below in section II.C. of this opinion, we conclude that it evidences the legislature's intent to preclude use of the seat belt defense in a contribution action.
¶ 30. Nor does the change effected by Wis. Stat. § 347.48(2m)(c) reveal any legislative intent to create a cause of action for contribution. It is important to recognize that a violation of subsection (2m)(c), the subsection imposing a duty upon drivers to "buckle up" their minor passengers, necessarily involves a violation of § 347.48(2m)(d), the subsection mandating use of a seat belt by minor passengers. If a person operates a motor vehicle without reasonably believing "that each passenger who is at least 4 years old and not more than 15 years old. . .is properly restrained," see § 347.48(2m)(c), it logically follows that "a person who is at least 4 years old. . .[is] a passenger in that motor vehicle [without being] properly restrained." See § 347.48(2m)(d).
¶ 31. "A basic rule of this court in construing statutes is to avoid such constructions as would result in any portion of the statute being superfluous." State v. Wachsmuth, 73 Wis. 2d 318, 324, 243 N.W.2d 410 [452]*452(1976). Read together with Wis. Stat. § 347.48(2m)(d) and (2m)(g), § 347.48(2m)(c) would be superfluous unless it had some additional purpose other than to reduce the plaintiffs recoverable damages by a maximum of 15%.6
¶ 32. In order to avoid a superfluous construction of the statute, we can identify two reasons for the inclusion of Wis. Stat. § 347.48(2m)(c). First, we conclude that the legislature included subsection (2m)(c) to ensure that defendants received, coupled with subsection (2m)(g), a possible 15% reduction in plaintiffs recoverable damages, even when the jury determines that the minor passenger is less than 15% negligent for failing to wear a seat belt. Accordingly, even if a jury attributes little negligence to the minor for failing to "buckle up," the driver is still negligent, and the passenger's damages may be reduced by a maximum of 15%.
¶ 33. A second purpose of Wis. Stat. § 347.48(2m)(c) is revealed by examining the interrelationship between Wis. Stat. §§ 347.48 and 347.50. The relevant provisions of the latter statute provide:
347.50 Penalties....
[453]*453(2m) (a) Any person who violates s. 347.48(2m)(b) or (c) and any person 16 years of age or older who violates s. 347.48(2m)(d) may be required to forfeit $10.
(b) No forfeiture may be assessed for a violation of s. 347.48(2m)(d) if the violator is less than 16 years of age when the offense occurs.
7
This statute clearly illustrates that the legislature did not want to impose penalties upon minors less than 16 years old for a violation of § 347.48(2m)(d), but would allow penalties to be imposed against "any person" who violates § 347.48(2m)(c). Therefore, subsection (2m)(c), coupled with § 347.50(2m)(a) and (b), also provides for the imposition of a $10 forfeiture against the driver alone for failing to properly restrain minor passengers between 4 and 15 years of age. By enacting subsection (2m)(c), the legislature has again acknowledged that the driver of an automobile is more responsible than a minor passenger for that minor's failure to "buckle up."
¶ 34. Most importantly, however, neither statutory change to the common law created an affirmative cause of action for contribution for that responsibility. Instead, a $10 penalty was authorized by Wis. Stat. § 347.50(2m)(a), and a § 347.48(2m)(g) 15% reduction in plaintiffs' recoverable damages was allowed even where the minor passenger is determined to be relatively faultless for failing to wear a seat belt.
¶ 35. Catholic Mutual cites Wis. Stat. § 347.48(4)(d) to support the proposition that the seat [454]*454belt defense may be used affirmatively in an action for contribution. It provides in relevant part:
(4) Child Safety Restraint Systems Required; Standards; Exemptions, (a) 1. No resident, who is the parent or legal guardian of a child under the age of 2, may transport the child in a motor vehicle unless the child is properly restrained in a child safety restraint system approved by the department....
(d) Evidence of compliance or failure to comply with par. (a) is admissible in any civil action for personal injuries or property damage resulting from the use or operation of a motor vehicle but failure to comply with par. (a) does not by itself constitute negligence.
¶ 36. Specifically, Catholic Mutual argues that the legislature's omission of the language "failure to comply.. .does not by itself constitute negligence" from Wis. Stat. § 347.48(2m)(g) and the inclusion of that language in subsection (4)(d) of the same statute "is a clear expression of legislative intent that a violation of subsection (2m) is a basis for liability." Catholic Mutual Brief at 8. We disagree.
¶ 37. As we have stated, since its recognition in 1967, the seat belt defense has never been interpreted by this court to provide grounds for an affirmative action against a third party. It has always been used to limit damages, not to compel the payment of damages. When the legislature adopted the common law seat belt defense in 1987, it did nothing to change that consistent and traditional characteristic of the seat belt defense.
[455]*455c.
¶ 38. Having determined that the seat belt defense has never before been employed as an affirmative cause of action for contribution, we are left to determine whether, as a matter of equity, defendants may be excused from liability to a greater extent than that allowed by Wis. Stat. § 347.48(2m)(g) when they are at fault for having caused the accident initially. We conclude that they may not.
¶ 39. We have previously recognized that the goal of ensuring safety through use of available seat belts is a laudable one. See Foley, 113 Wis. 2d at 489 ("We hope that passengers will also be encouraged to wear seat belts if their potential compensation for injuries is reduced."). However, this goal, as with any principle of equity, must be balanced against the unquestionably sound goal of ensuring public safety through safe and attentive driving on the state's highways and streets. We cannot completely overlook Holcka's negligent conduct in assessing Gaertner's negligent failure to restrain Koldeway in a seat belt.
¶ 40. The facts of this case illustrate why we are not persuaded that the seat belt defense can be used as an affirmative action for contribution. Were we to allow Catholic Mutual's claim to proceed in this case using their own formula for contribution, Gaertner (0% negligence in causing the accident) would be responsible for over $308,000 of Koldeway's injuries, leaving Holcka (100% negligence in causing the accident) responsible for less than $192,000 of Koldeway's approximate $588,000 in total damages. Although Gaertner had an insurance policy worth only $100,000 in this case, we cannot presume that this will always be the case; nor can we assume that defendants' insurers will, as Catholic Mutual has done here, release plaintiffs in [456]*456Gaertner's position from liability above their policy limits.
¶ 41. Although the seat belt defense does not, strictly speaking, affect the determination of causal negligence in any action for personal injury, this case illustrates that use of the seat belt defense in an affirmative cause of action for contribution can drastically alter the landscape of liability by reducing defendants' overall financial responsibility, regardless of the amount of fault that is attributable to the defendant for causing the accident initially.
¶ 42. Such a policy determination would require this court to declare that seat belt negligence effectively outweighs or supersedes the active causal negligence in any automobile accident. Regardless of the defendant's responsibility for causing the original accident, the driver's failure to properly restrain a passenger could almost eliminate the defendant's financial responsibility altogether. On the other hand, there may be a point when active causal negligence carries more weight than seat belt negligence — a point at which the driver's liability for the passenger's injuries should cease. Performing this balance would thrust this court into a policy-making role more appropriately left to the legislature.
¶ 43. More importantly, as we have mentioned, the legislature has explicitly declared that one's own seat belt negligence should not outweigh the determination of active causal negligence in an automobile accident — a possibility that clearly remained after our decision in Foley. Instead, the legislature has indicated that seat belt negligence may only reduce the injured party's incremental injuries by a maximum of 15%. See Wis. Stat. § 347.48(2m)(g). We conclude that in doing [457]*457so, the legislature also intended to limit the potential windfall to defendants who are determined to be causally negligent. We decline to hinder that stated policy by allowing the present cause of action for contribution to proceed.
III.
¶ 44. Because we conclude that Wis. Stat. § 347.48(2m)(g) evidences the legislature's intent to bar claims for contribution involving seat belt negligence, we affirm the order of the circuit court which dismissed Catholic Mutual's motion for summary judgment against American Family.
By the Court. — The order of the circuit court is affirmed.