I've had two bike accidents, both while touring overseas, and neither involved a motor vehicle. The first took place 25 years ago, when helmets were still fairly rare. I wasn't wearing a helmet. The accident was caused by a broken fender/mudguard. All I know about the accident is what my friend later told me. I went flying over the handlebars while going down a hill in western Ireland. I had a serious concussion, had amnesia for 48 hours, and was hospitalized for 2 nights. My face was stitched up in 3 places.
I began wearing a helmet when I returned home after that accident, and in the early 1980s, I was often openly laughed at or stared at when biking outside of North America. There was a time not too many years ago when only Americans, Canadians, Kiwis, & Aussies wore helmets.
Years later, I was descending a long hill on the Great Ocean Road in Australia, pulling a Burley trailer carrying my friends' one year old boy. At the bottom of the hill around a blind curve was a bridge with a wooden plank surface. The old planks had partially rotted away and there were many wide gaps in the road surface, one of which immediately swallowed my front wheel. I had been breaking as much as I could before my wheel was caught. I was unable to cross the bridge diagonally to prevent from getting caught in a gap because there happened to be a car coming in the other direction. I flew over the handlebars. The clever trailer hitch design worked precisely as intended and the trailer stayed upright and the toddler was unharmed. His mother told me my helmet lightly hit the side of the bridge, but I didn't feel anything, nor could I see any damage to the helmet. My front wheel was pretzeled and I had a broken rib or two, which didn't keep me from continuing the tour once I had replaced the wheel.
About 5 years ago, my helmet served me well in an unexpected way. A friend & I were touring in Tunisia. In several isolated villages in different parts of the country, young boys harassed us in different ways such as throwing stones at us, attempting to block us in the road, and even trying to pull us down while riding. In one small oasis village just north of the Sahara, a boy launched a huge date palm branch into the air, and it came down on my helmet. I was unharmed.