Vancouver has banned doorknobs through it's building code. This article is fascinating.
The motive is for greater accessibility. And I can think of several reasons why levers offer a lot more affordance. Elbows, forearms, feet, items gripped in a hand, butts can all be used to open doors with handle knobs... but that is or will be discussed. I recommend this fascinating article on the topic.

**[Vancouver’s ban on the humble doorknob likely to be a trendsetter][1]**


  [1]: http://www.vancouversun.com/story.html?id=9173543