BOSLAUGH, J.

This is an action for wrongful death brought by the administrator of the estate of Dennis E. Brugh, deceased, against Owen D. Peterson and Lloyd K. Fischer. The jury returned a verdict in the amount of $35,000 against each defendant. The defendants' motions for judgment notwithstanding the verdict were overruled, but their motions for new trial were sustained. The plaintiff has appealed and the defendants have cross-appealed.

The plaintiff's decedent, Dennis, died as the result of injuries sustained in an automobile accident that occurred near Lincoln, Nebraska, at about 10 p.m. on March 12, 1966. The weather was clear and the road was dry. Dennis was riding in a 1965 Pontiac station wagon owned by the defendant Fischer and operated by his son Kenneth. The Fischer automobile was proceeding east on Pioneers Boulevard and had entered the intersection at Seventieth Street when it was struck by a 1965 Plymouth automobile operated by the defendant Peterson and proceeding south on Seventieth Street.

Pioneers Boulevard east of the intersection was a gravel road. The intersection, Seventieth Street, and Pioneers Boulevard west of the intersection are surfaced with black-top. Seventieth Street from the north has a slight incline, approximately 2 percent, as it approaches the intersection. Pioneers Boulevard slopes down as it approaches the intersection from the west, and an embankment along the north edge of Pioneers Boulevard gradually diminishes toward the intersection. Seventieth Street is an arterial protected by stop signs on each side of the intersection. There is a "Stop Ahead" sign on Pioneers Boulevard approximately 400 feet west of the stop sign and a "Pavement Ends" sign approximately 1,000 feet west of the intersection. The posted speed limit on Seventieth Street is 45 miles per hour.

Kenneth Fischer, who was driving the Pontiac station wagon, has no recollection of the accident. The last thing he remembers is turning east onto Pioneers Boulevard from Forty-eighth Street. Linda Weiderspan, a passenger in the Fischer automobile, also has no memory of the accident.

Frank Campbell testified that he was sitting on the right side of the front seat of the Fischer automobile at the time the accident happened. Campbell also testified that Kenneth Fischer intended to turn north on Seventieth Street and was going about 30 or 35 miles per hour; that Campbell did not see the "Pavement Ends" sign or the "Stop Ahead" sign; that he is not sure that he saw the stop sign but he knew that it was there; that he said to Fischer, "`I think we passed a stop sign back there'"; that Fischer said, "`Did I?'"; that the impact then occurred; and that the automobile began to spin. On cross-examination Campbell testified that the Fischer car had entered the intersection when he said that he thought they had passed a stop sign.

The defendant Owen Peterson testified that he was driving the 1965 Plymouth automobile south on Seventieth Street at approximately 45 miles per hour; that he first saw the Fischer automobile when he was 75 to 100 feet north of the intersection; that the Fischer automobile was then about the same distance west of the intersection; that he applied his brakes immediately; that the Fischer automobile proceeded into the intersection without stopping or slowing down; and that the impact occurred.

The front of the Peterson car struck the left side of the Fischer automobile near the rear wheel. The point of impact was in the west lane of Seventieth Street near the center of the intersection. The Peterson automobile left skid marks of from 24.5 feet to 56 feet leading up to the point of impact. There was extensive damage to the front end of the Peterson automobile and to the left side of the Fischer automobile.

The Fischer automobile came to rest in the ditch southeast of the intersection, headed to the southwest, with its left side against a utility pole. The left front wheel of the Fischer automobile was 47 feet from the center of the intersection.

The Peterson automobile came to rest on the east shoulder of Seventieth Street, headed to the, south. The left rear wheel of the Peterson automobile was 63 feet from the center of the intersection.

The plaintiff's decedent was riding as a guest in the Fischer automobile. Before the plaintiff could recover from the defendant Fischer it was necessary to establish gross negligence in the operation of the Fischer automobile. 39-740, R.R.S. 1943.

Gross negligence within the meaning of the motor vehicle guest statute means gross and excessive negligence or negligence in a very high degree; the absence of slight care in the performance of duty; an entire failure to exercise care; or the exercise of so slight a degree of care as to justify the belief that there was an indifference to the safety of others. Callen v. Knopp, 180 Neb. 421, 143 N.W.2d 266.

The failure to stop at the stop sign or see the Peterson automobile, alone, was not sufficient to establish gross negligence. Callen v. Knopp, supra. The failure to observe the warning signs did not establish gross negligence. Boismier v. Maragues, 176 Neb. 547, 126 N.W.2d 844.

There was evidence of a warning from the passenger Campbell, but this occurred after the Fischer automobile had entered the intersection and was just before the impact. There was evidence of testimony by Campbell at a previous hearing in another proceeding concerning an earlier warning, but this evidence was admitted only for the purpose of impeachment.

There is no evidence in this case of negligent conduct extending over a period of time. The negligence of Fischer is characterized as momentary in nature. As we view the record, the evidence failed to establish gross negligence. The motion of the defendant Fischer for judgment notwithstanding the verdict should have been sustained.

With respect to the defendant Peterson, the plaintiff alleged that Peterson operated the Plymouth automobile at an excessive speed; that he failed to reduce its speed; that he failed to keep a proper lookout; and that he failed to maintain a reasonable control.

On cross-examination the defendant Peterson admitted that he had stated, in a previous deposition, that he had not looked to the left or to the right as he approached the intersection. Although this admission was subject to explanation and interpretation, it tended to support the plaintiff's allegation of the failure to keep a proper lookout.

On the issue of speed the plaintiff produced an expert witness, David I. Cook, an Associate Professor of Engineering Mechanics. Professor Cook was allowed to testify, over objection, in answer to a hypothetical question, that the Peterson automobile was traveling at not less than 52 miles per hour before the accident.

Professor Cook testified that he had determined a coefficient of friction by experimentation; and that upon the basis of the skid marks made by the Peterson automobile prior to the impact, it had a minimum speed of not less than 30 miles per hour if it was assumed that the Peterson automobile had stopped at the point of impact. This evidence was proper. A qualified expert, upon laying a proper foundation, may give his opinion as to the minimum speed which a vehicle must have been traveling to lay down the skid marks shown in the evidence. Flory v. Holtz, 176 Neb. 531, 126 N.W.2d 686.

Professor Cook's determination of the speed of the Peterson automobile in this case, including the impact, involved an application of the law of conservation of energy, the law of conservation of momentum, and the law of recovery. It was essentially a mathematical calculation in which it was necessary to determine a large number of factors and variables. Some were determined by experimentation. Others were determined by inspection of photographs taken at the scene of the accident. Others were determined by assumption.

As an example, Professor Cook assumed that both drivers had no control over their vehicles after the impact. There was no direct evidence concerning this factor, a factor which could be of great significance because direction and distance traveled after the impact are of importance in the calculation made.

The situation in this case is comparable to that which was presented in Flory v. Holtz, supra. There we held that similar testimony by this same witness was properly excluded. The case points out that the foundation for the opinion was inadequate, that it depended in part upon inferences based upon assumptions, and, that there were many unknown factors which might have influenced the result.

The evidence here is such that the jury might infer that the Peterson automobile was traveling at an excessive speed as it approached the intersection. Flory v. Holtz, supra. But the expert opinion depends upon the resolution of so many variables that it is, in effect, a statement of a possibility. Under the circumstances in this case the expert testimony was neither necessary nor advisable as an aid to the jury. See Drahota v. Wieser, ante p. 66, 157 N.W.2d 857.

We think that the trial court erred in permitting Professor Cook to state his opinion as to the speed of the Peterson automobile in this case, including the impact, and that his testimony should have been confined to speed based upon skid marks only.

Some criticism is made of instruction No. 13 relating to the duty of a driver to yield the right-of-way if necessary to avoid a collision. Perhaps a more accurate statement of the rule is that, although a driver may have the technical right-of-way, if the situation is such as to indicate to the mind of an ordinarily careful and prudent person in his position that to proceed would probably result in a collision, then he should exercise ordinary care to prevent an accident, even to the extent of waiving his right-of-way. See Maska v. Stoll, 163 Neb. 857, 81 N.W.2d 571.

It is unnecessary to consider the other assignments of error.

The order of the district court sustaining the motion of the defendant Peterson for a new trial is affirmed. The order of the district court sustaining the motion of the defendant Fischer for a new trial is reversed and the cause remanded with directions to sustain the motion of the defendant Fischer for judgment notwithstanding the verdict.

AFFIRMED IN PART, AND IN PART REVERSED AND REMANDED WITH DIRECTIONS.

SMITH, J., participating on briefs.