Especially on a sunny day in the winter or spring, I often notice faint rainbows when I look down at the edge of the road by the curb. I am not sure what causes these rainbows, but I believe that they are caused by light reflecting off tiny salt crystals that have accumulated on the ground.
Why do rainbows form under such conditions? The standard mathematical explanation for rainbows assumes spherical raindrops for simplicity, and for water droplets suspended in air, that seems like a reasonable approximation. But salt crystals (or whatever particles on the ground that are causing the rainbows) have irregular shapes and are dispersed randomly on the ground. Wouldn't such random configurations preclude a reflection pattern as coherent as a rainbow?
EDIT: To help clarify what I'm talking about, here is a short video of the type of rainbow effect that I have observed. (You might want to turn off the sound; unfortunately I don't have a video editing program to remove the sound.)
EDIT: The accepted answer below mentions retroreflector glass beads. I took a sample from the ground, which a friend of mine took a close-up photo of as well as a flash photo of. Both photos support the glass bead theory.
Best Answer
I suspect the rainbow is from retroreflector glass beads rather than salt. Tiny glass beads are placed on the surface of road markings, halfway embedded, making them reflect light from vehicles back to them with total internal reflection. But when they get loose they act as cloud droplets instead, making rainbows.