I'm completely unsurprised as to how Istanbul is at the top of the list. Istanbul is an ancient city on top of 7 steep hills. It's also the largest city in Europe and the 5th largest in the world. Istanbul even particularly in the slightly older parts of town (by which I mean only 500-600 years old, not the 3000 year old bits) was mainly built for tall buildings and narrow roads for hand and horse carts, not for cars and trucks.

In the winter it snows quite heavily in Istanbul, meaning that all of the hills tend to get cars that aren't properly chained sliding down the hill to the bottom.

I've often joked that the main reason there aren't more fatal traffic accidents in Istanbul is nothing to do with the way people drive (which is extremely aggressively by western standards) but more to do with the fact that it's hard to sustain a speed over 30 mph in most of the city for any real length of time.