A revolving door is never open. That is the whole invention. Theophilus Van Kannel patented it in Philadelphia in 1888 as a “storm door”, and the trick is that the wings always seal against the curved walls, so however busy the entrance gets, there is never a straight path for the wind from street to lobby. Everyone gets through and the weather never does. In 2006 some MIT students counted who used the one in a campus building: about a quarter of people. The rest took the ordinary hinged door beside it, which let in up to eight times more air. Given a door that is always shut and a door that is honestly open, most of us pick the honest one, even with our hands full.