Scott Berube appeals from the judgment of conviction entered in the Superior Court (York County, Delahanty, C.J.) on a jury verdict finding him guilty of Class A manslaughter, 17-A M.R.S.A § 203(1)(A) (Supp. 1994). Berube does not include among his claims of error any challenge to the trial court’s instructions to the jury. We conclude, nevertheless, that the court’s failure to instruct the jury on a partial defense generated by the evidence at trial constitutes obvious error requiring that the judgment be vacated.
On July 16,1992, while riding his motorcycle, Berube struck and killed 85-year-old Florence Downs as she crossed Route 109 in Sanford. Berube was indicted and tried for manslaughter. The central issue at trial was whether the speed at which Berube was traveling at the time of the accident constituted reckless or criminally negligent conduct. The posted speed limit was 45 mph. The State’s accident reconstructionist testified that Berube was traveling at a minimum speed of 78 mph. Berube’s accident recon-structionist testified that his speed was in a range of 59 to 63 mph. Berube testified that he was going between 50 and 60 mph. Three eyewitnesses testified that he was traveling at 80 to 100 mph. The passenger in the ear behind Berube testified that Berube was going 80 to 85 mph. The jury found Berube guilty of Class A manslaughter. The court entered a judgment on the verdict and sentenced Berube to 12 years confinement.
Notwithstanding Berube’s failure to raise the issue at trial or on appeal, we review the court’s failure to instruct the jury on a statutory defense “in order to maintain the basic integrity of judicial proceedings” and “to avoid depriving the defendant of his constitutional right to a fundamentally fair trial.” State v. Bahre, 456 A.2d 860, 864 (Me.1983).1
The State charged Berube with manslaughter pursuant to 17-A M.R.S.A § 203(1)(A) alleging that he recklessly, or with criminal negligence, caused the death of Downs. The statute provides a partial defense in cases of vehicular homicide:
3. Manslaughter is a Class A crime except that:
A. It is a defense to a prosecution of a manslaughter based upon the reckless or criminally negligent operation of a motor vehicle, which reduces the crime to a Class B crime, that the death of the victim resulted from conduct that would otherwise be defined only as a civil violation or civil infraction.
17-A M.R.S.A § 203(3) (Supp.1994). Speeding is a civil offense unless the vehicle exceeds the speed limit by 30 miles an hour or more. 29 M.R.S.A. § 1252(4) (1978) replaced by 29-A M.R.S.A. § 2074(3) (Pamph.1994) (effective Jan. 1, 1995). In this case the State conceded at trial that its theory of criminal liability rested solely on excessive speed and that if it were found that Berube’s speed did not exceed the speed limit by 30 miles per hour then he could not be guilty of Class A manslaughter. As discussed above there was ample evidence in this case to generate the defense because two witnesses testified that Berube was traveling less than 30 mph over the speed limit. Nevertheless, neither did Berube request an instruction on *172this defense, nor did the court give one sua sponte.
Maine’s criminal code requires the State to disprove beyond a reasonable doubt any statutory defense “in issue as a result of evidence admitted at the trial which is sufficient to raise a reasonable doubt on the issue.” 17-A M.R.S.A. § 101(1) (1983). “The State’s obligation to disprove a defense generated by the evidence is the functional equivalent of the State’s burden to prove all of the elements of the offense.” State v. Begin, 652 A.2d 102, 106 (Me.1995). In Begin, we reiterated that “the nature of the criminal process requires that the State disprove the defense, even though the defendant made a tactical decision not to assert the defense.” Id. (quoting State v. Grover, 460 A.2d 581, 584 (Me.1983)). Moreover, obvious error results when the court fails to instruct the jury on a statutory defense generated by the evidence. Begin, 652 A.2d at 106. We accordingly conclude that it was obvious error for the trial court in this case to fail to instruct the jury on the section 203(3)(A) defense.2
We find no merit in Berube’s other claims of error. Contrary to his contention, the State did not commit a discovery violation when it failed to furnish him with a witness’s oral statement made in the course of trial preparation. State v. Whitten, 499 A.2d 161, 162 (Me.1985). Furthermore, we discern no obvious error in the court’s decision to permit Downs’s daughter to testify and its failure to give sua sponte a curative instruction when she began to cry on the stand.
Finally, the court did not abuse its discretion when it permitted three eyewitnesses to give an opinion in miles per hour about the speed at which the motorcycle was traveling. All three viewed the motorcycle from stationary vantage points and in relation to a car that was following it. All had previously viewed motorcycles and other ve-hides speeding down Route 109 on many occasions. Moreover, each had a substantial familiarity with motorcycles that would provide a basis for rationally considering the sound of the motorcycle as an indicator of speed. As a result, these particular witnesses had an opportunity to acquire an “intelligent thought” about the speed of Be-rube’s motorcycle, and the court did not err by permitting them to so testify. Cf. Wiles v. Connor Coal & Wood Co., 143 Me. 250, 261, 60 A.2d 786, 792 (1948) (view of vehicle split-second before impact with victim by victim’s child companion while crossing street would not permit “intelligent thought” about vehicle’s speed).
Finding no error besides the failure to instruct on the section 203(3)(A) defense, we conclude that Berube is not entitled to a new trial. Section 203(3)(A) acts as only a partial defense to Class A manslaughter and is considered by the factfinder only after the factfinder determines that the State has proved beyond a reasonable doubt that the defendant is guilty of manslaughter. A failure to instruct on the defense, therefore, does not effect the determination of guilt in the first instance. Even absent the instruction, Berube received a full and fair trial on whether he recklessly, or with criminal negligence, caused the death of Downs. The jury answered affirmatively, thus leaving unresolved only the issue of whether Downs’s death resulted from conduct that would otherwise be defined only as a civil violation or civil infraction. Berube thus properly faced a situation in which the outcome most favorable to him was a conviction on Class B manslaughter.
Ordinarily we would vacate the judgment and remand for a new trial, thereby permitting the State another opportunity to obtain a conviction on Class A manslaughter. In this case, the State has offered to accept a judgment of conviction on Class B manslaughter in lieu of retrying the case. Be*173cause this result does not unfairly prejudice Berube, we vacate the judgment and remand for an entry of judgment of Class B manslaughter.
The entry is:
Judgment vacated. Remanded with instructions to enter a judgment of conviction of Class B manslaughter and to resentence the defendant accordingly.
WATHEN, C.J., and ROBERTS, GLASSMAN, and RUDMAN, JJ., concurring.
. At the request of the Court, both parties submitted supplemental briefs addressing this issue.
. The State argues that this case is distinguishable from Begin because Berube "voluntarily, competently, intelligently, and knowingly waived” an instruction on that defense. Although the record indicates that Berube made no requests for special instructions or a special verdict form, the record before this Court does not contain a waiver of an instruction on the section 203(3)(A) defense. We do not infer a waiver from a defendant’s strategy. See State v. Begin, 652 A.2d 102 (Me.1995) (where the defense of "it never happened” was arguably inconsistent with the defense of voluntary consumption of drugs).