MEMORANDUM OPINION
BROCK, C.J.The plaintiffs, John and Susan Meaney, appeal the Superior Court (Sullivan, J.) denial of their motion to set aside the verdict and for a new trial based on the trial court’s failure to instruct the jury that brake failure is an affirmative defense. We affirm.
On June 10, 1992, plaintiff John Meaney was exiting his State Police cruiser at a construction site on Interstate 89 in Hopkinton when a vehicle operated by the defendant, Alfred Rubega, struck the cruiser. The plaintiffs sued the defendant for negligence and for loss of consortium. The defendant denied liability “on the basis of the failing of his brakes, which were presumed to be in good working order.”
At trial, the plaintiffs requested that the trial court instruct the jury that brake failure is an affirmative defense, requiring the defendant to prove by a preponderance of the evidence that brake failure was the cause of the accident. The trial court denied the request. The jury returned a verdict for the defendant; this appeal followed.
*532“The purpose of jury instructions is to identify the factual issues which are material for a resolution of the case, and to inform the jury of the appropriate standards by which they are to decide them.” Peterson v. Gray, 137 N.H. 374, 377, 628 A.2d 244, 246 (1993) (quotation omitted). We review the propriety of jury instructions by looking at the charge as a whole. Chellman v. Saab-Scania AB, 138 N.H. 73, 79, 637 A.2d 148, 151 (1993). “A jury must be instructed adequately and accurately on the relevant law, and whether or not a particular instruction is necessary is left to the trial court’s sound discretion.” State v. Seymour, 140 N.H. 736, 744, 673 A.2d 786, 793 (1996).
An affirmative defense is a “matter asserted by [the] defendant which, assuming the complaint to be true, constitutes a defense to it.” BLACK’S LAW DICTIONARY 60 (6th ed. 1990). In other words, by raising an affirmative defense, the defendant admits the plaintiff’s allegations but provides an excuse or justification or otherwise defeats the plaintiff’s cause of action. See Seavy v. Dearborn, 19 N.H. 351, 355 (1849); see also Fothergill v. Seabreeze Condos., 141 N.H. 115, 116-117, 677 A.2d 696, 697-98 (1996) (noting statute of limitations is affirmative defense).
On the other hand, “a denial is a traverse of the facts alleged in the [plaintiff’s] pleading.” 61A AM. JUR. 2D Pleading § 169 (1981). Rather than accept the plaintiff’s characterization of the facts, a general denial “goes to the root of the cause of action and permits the introduction of any proper evidence tending to controvert the facts which the plaintiff must establish to sustain his [or her] case.” Id. § 364.
The defendant’s assertion of brake failure as the cause of the accident was a denial of negligence, not an affirmative defense. The plaintiffs alleged that the defendant “failed and neglected to keep a proper lookout and to operate his vehicle in a safe, reasonable, lawful, and prudent manner.” Rather than agreeing with the plaintiffs’ characterization, the defendant contested the plaintiffs’ assertion that he operated his vehicle in a negligent manner. The cause of the accident, the defendant maintained, was brake failure, not his negligence. Although brake failure “sounds like a defense, ... it reflects nothing more than a denial by the defendant of negligence.” Dyer v. Herb Prout & Co., 126 N.H. 763, 765, 498 A.2d 715, 717 (1985); accord McVey v. St. Louis Public Service Company, 336 S.W.2d 524, 528 (Mo. 1960) (stating brake failure not affirmative defense but evidence from which jury could have found that defendant was not negligent).
*533A defendant charged with the negligent operation of a motor vehicle may show himself free of the alleged negligence by evidence of a sudden brake failure. Such evidence does not present an affirmative defense but goes to show that the sudden brake failure, and not the defendant’s negligence, was the proximate cause of the injury. Sudden brake failure ... is merely a denial of the plaintiffs’ cause of action and is properly received under a general denial. The risk of non-persuasion on the issue of defendant’s negligence remains with the plaintiff.
Coffel v. Spradley, 495 S.W.2d 735, 740 (Mo. Ct. App. 1973) (citations omitted).
The trial court properly rejected the plaintiffs’ contention that brake failure is an affirmative defense. Consequently, we find no abuse of discretion in the trial court’s refusal to instruct the jury otherwise. See Seymour, 140 N.H. at 745, 673 A.2d at 793.
Affirmed.
JOHNSON, J., dissented; the others concurred.