Plaintiff recovered a judgment for damages suffered in an intersection collision, and the defendant appeals. The same litigation has been in this court before. Lundquist v. Irvine, 243 Or 274, 413 P2d 416 (1966).
The defendant’s principal assignment of error attacks an instruction that the defendant was guilty of negligence as a matter of law in failing to signal her intent to turn her automobile into the path of the plaintiff’s automobile and that such “negligence was a proximate cause of the collision.”
The defendant admitted that she had not made a turn signal. The collision occurred at a five-point intersection. All traffic entering the intersection from the street upon which the defendant was driving had to make either a 45-degree turn to the right or a 45-degree turn to the left. Those vehicles turning toward their right would not interfere with traffic proceeding through the intersection from other directions, but those turning toward their left would have to move across traffic lanes that might be occupied by other vehicles. In the circumstances, a left turn was a movement of the type for which a signal was mandatory. ORS 483.126. The collision was of the kind that á proper signal would have been likely to avert. There *496was, therefore, no error in the instruction that the failure to signal was negligence as a matter of law.
Taking the question of causation from the jury, however, was for the trial court a closer question. In some cases, where only one act of a defendant can be identified, its causal relation to the harm may be clear, and the jury need consider only the question of negligence. Thus, if A shoots B with a rifle, and the jury finds that he did so negligently, the cause of the harm is obvious, and there would be no error in taking from the jury the question of causation. But ordinarily, if reasonable persons could disagree about the cause of a particular harm, the question is for the trier of fact. Babler Bros. v. P. I. E. Co., 244 Or 459, 415 P2d 735 (1966).
In the case at bar, a number of acts of each of the two drivers had been alleged to have been negligent, and one or more of these acts may or may not have contributed to the cause of the collision. These issues were made by the pleadings, and there was evidence in the record to support the allegations that were submitted.
In telling the jury that the defendant’s failure to signal was both negligent and causative as a matter of law, the trial court ruled out the possibility that the collision might have occurred whether or not the defendant had made a proper signal. Before making such a ruling, a trial court ought to be very certain that it is not usurping a function of the jury.
The plaintiff swore that if the defendant had signaled her intention to turn the plaintiff would have seen the signal, and would have been prepared for her maneuver. But the question of the plaintiff’s own negligence with reference to speed, lookout, and control *497was also in the case. While the failure to signal may have been at least one canse of the collision, the court should not have taken from the jury the right to find otherwise. In so ruling, the trial court was holding as a matter of law that reasonable persons could not draw any other inference than the one the court was drawing. Since there was some conflict in the testimony concerning the conduct of the respective drivers, the drawing of inferences about the cause of the collision should have been left to the jury.
Reversed and remanded.