Gary Hustwit concludes his trilogy on design with “Urbanized,’’ a look at how cities are shaped. His examples run from the slums of Mumbai, where there’s one toilet for every 600 people, to New York’s wildly popular High Line, an abandoned elevated rail line turned into a leafy park.

By 2050, some 75 percent of the world’s population will live in cities. It’s a frightening prospect, and Hustwit calls on architects, planners, builders and activists for advice. The documentary at times is weighed down by statistics and shop talk, but Luke Geissbuhler’s smartly rendered cinematography saves the day.