That yield sign is at the edge of a park, and part of the Willamette Valley Scenic Bikeway, and thus there may be a higher than expected volume of bicycles, and pedestrians proceeding straight while all cars turn, so perhaps there were traffic conflicts there in the past.
An alternative option would be to trigger bike/pedestrian lights in an independent light cycle to vehicle movements. I had posted this a while ago that when the crosswalk was activated, the cars got a no right on red light.
Street Signs
However, one still has to deal with bicycles on the road. I think there was a bike path there.