A retroreflective article and method is provided. The retroreflective article includes a first transparent layer having a back surface with retroreflective prisms, and a planar front surface that has an index of refraction n1. A second transparent layer having an index of refraction n2 is less than n1 overlies the back surface of the first layer. The second layer includes an array of uniform nanostructures of transparent material that define pores having a width that is less than one half of the wavelength of visible light. The nanostructures may be an array of parallel, same-sized nanorods that are obliquely oriented with respect to the first transparent layer, or a grid-like or parallel array of tapered nano-ridges of uniform size that define tapered pores or tapered grooves. The second layer preferably has a porosity greater than 60% so that the resulting index of refraction n1 is no larger than about 1.20.