I think this would be a good feature for a traffic simulator. It used everywhere on streets and vehicles. And is not possible to properly mimic with current materials.
Were current reflection more or less works like a mirror. Retro reflection offcourse works by the fact the outgoing ligth is bounced in the same direction as the incoming ligth.
This video show very nice how it differs form normal reflective shaders/materials:
Forexample: signs, lightclusters, warning markers etc.
I think such a material could use the alphachannel to specify the amount of retro reflection similair how normal materials do this. (black is no reflection, white full)
Maybe it would be a nice touch for some future versions.
