Here is a list of wildflowers Georgia DOT workers have planted on roadsides over the years:
Golden Wave coreopsis
Showy primrose is not something that I'm going to plant, but it's stunning in the median of a four-lane road.
My personal favs from this list are Gaillardia, Coneflower (Echinacea), Black eyed Susans and corn poppies. I have seeds of Mexican hats and cornflowers saved from last spring, too.
I'll be planting silene and Madagascar periwinkles, verbena bonairensis and some others that are seldom on a 'wildflower list.'
I'll add snapdragons, larkspur, nicotiana, globe amaranth, saliva coccinea, breadseed poppies, Iceland poppies and California poppies -- not all on the roadside and not all in the same bed. Some are seeded now, some in the spring. Periwinkles are reliable self-reseeders as are several others.