Less a documentary than a meditation on Detroit’s slow descent, Detropia looks at how the fastest-growing city in the world 80 years ago became America’s fastest-shrinking metropolis today. The primary culprit is obvious: The auto industry has largely moved its production where labor is cheaper. Directors Heidi Ewing and Rachel Grady (the team behind the fantastic Jesus Camp) are observational filmmakers who, unlike a Michael Moore or an Errol Morris, don’t actively engage with their subjects in the final edit. That gives Detropia many perspectives other than those of the filmmakers, which makes the documentary feel a little too unplanned at times; it never establishes a true finish line. Detropia provides some interesting background, but doesn’t offer light at the end of the tunnel.