On what side of the escalator do people stand in the U.S.?

I've never paid any attention to it when I've been there - but somehow I got the impression that they stood on the left, because of the signs in the London tube that say to stand on the right (which I always assumed were there for the same reason as the 'LOOK RIGHT' painted on the roads - viz. for the benefit of visiting U.S.ese). But on Canadian escalators people definitely stand on the right as in the U.K...