Alexander, J. Petrich, A.c.j., and Morgan, J., concur.
Richard J. Payne appeals a partial summary judgment of the Cowlitz County Superior Court, denying him the affirmative defense of contributory fault. The defense arose from Lynn Clark's alleged failure to use a safety belt. We affirm.
The facts are not in dispute. A car accident occurred on May 10, 1987, on Interstate 5 in Cowlitz County. A vehicle driven by Payne struck Clark's vehicle from behind causing Clark's vehicle to spin out of control and go off the roadway. As a result of the accident, Clark suffered severe injuries.
Clark sued Payne for damages. Although Payne admitted negligence, he raised as an affirmative defense Clark's contributory fault for failure to wear a seat belt. Clark had previously admitted that her vehicle had fully functional safety belts and had acknowledged that she chose not to wear hers.
The parties agreed that $400,000 was fair and reasonable compensation for Clark's general and special damages. Clark then moved for partial summary judgment. She requested that the trial court dismiss the affirmative defense, and claimed that the so-called "seat-belt defense" was unavailable to offset or reduce the amount of damages owed. The case was tried to the court on stipulated facts and a judgment of $400,000 was awarded to Clark with no reduction for Clark's failure to wear a safety belt.
The sole question on appeal is this: Did the trial court err in granting partial summary judgment denying the affirmative defense of contributory fault for failure to avoid injury or to mitigate damages arising from failure to wear a safety belt?
Payne argues that the mandatory safety belt statute*fn1 bars introduction of safety belt evidence only to show negligence, but does not bar that evidence to show contributory fault. We disagree.
The contributory fault statute, RCW 4.22, defines fault as "acts or omissions . . . that are in any measure negligent or reckless . . .". The definition goes on, however, to say that "[t]he term also includes breach of warranty, unreasonable assumption of risk, and unreasonable failure to avoid an injury or to mitigate damages." RCW 4.22.015. Payne's position is that the additional language allows for introduction of evidence of the plaintiff's fault not amounting to negligence. He specifically argues that the defense of unreasonable failure to avoid injury or to mitigate damages is not a contributory negligence defense, but rather a defense of contributory fault and that evidence supporting such a defense would, therefore, be admissible even though evidence of negligence is barred.
 To resolve this appeal, we turn first to principles of statutory construction. The court's primary objective is to ascertain and implement the intent of the Legislature by the language of the statute. State v. Von Thiele, 47 Wash. App. 558, 562, 736 P.2d 297 (citing Service Employees Int'l Union, Local 6 v. Superintendent of Pub. Instruction, 104 Wash. 2d 344, 348, 705 P.2d 776 (1985)), review denied, 108 Wash. 2d 1029 (1987). If the statutory language is plain and unambiguous, the court's inquiry must end because a statute's meaning must be derived from the wording of the statute itself. Service Employees Int'l Union, Local 6 v. Superintendent of Pub. Instruction, supra (citing
Human Rights Comm'n v. Cheney Sch. Dist. 30, 97 Wash. 2d 118, 121, 641 P.2d 163 (1982). Here, RCW 46.61.688(6) clearly prohibits the plaintiff's failure to use a safety belt as evidence of negligence in a civil case. What is less clear is whether the Legislature intended the definition of negligence within that statute to be synonymous with the definition of fault found in RCW 4.22.015.
[2-4] RCW 46.61.688 does not define negligence. Where unambiguous words are not defined within a statute, the court will usually ascribe to those words their plain and ordinary meaning. Intent, if ascertainable, may be of assistance but cannot override an otherwise discernible plain meaning. Washington Pub. Util. Dists.' Utils. Sys. v. PUD 1, 112 Wash. 2d 1, 6-7, 771 P.2d 701 (1989); North Coast Air Servs., Ltd. v. Grumman Corp., 111 Wash. 2d 315, 320-21, 759 P.2d 405 (1988). Negligence is defined as a "failure to exercise the care that a prudent person usu[ally] exercises". Webster's Third New International Dictionary 1513 (1986). In addition, a plaintiff's negligence relates to a failure to use due care for his own protection. Seattle-First Nat'l Bank v. Shoreline Concrete Co., 91 Wash. 2d 230, 238, 588 P.2d 1308 (1978). Consequently, a plaintiff's self-directed negligence may justify reducing his recovery in proportion to his degree of fault. Seattle-First, 91 ...