As yet another person tripped over the back of my chair in yet another packed San Francisco restaurant, I finally realised that the problem was mine, not theirs. For comfortable sitting the body-hip angle is supposed to be 90-120 degree. Every chair here seems to be a kind of bucket. For a few seconds I wondered why the San Franciscans put up with such discomfort. However, I soon noticed the other patrons and the simple fact that, with a fat enough arse, thighs are raised at the arse end, but not much at the knee end, such that the correct angle is maintained.
2 days ago