Will you be purchasing seed or just letting what's there grow? I'm not familiar with the area you decribed but have seen the wildflowers along the roadsides in southern NC. I'm not sure, but I would bet those are a carefully selected mix of native grasses and wildflowers that all bloom around the same time. The land was most likely treated with herbicide, plowed and then seeded with the mix. I've seen them mowing these areas as soon as the flowers had gone to seed. I've not seen them collecting it, but would not be surprised. Good seed is VERY expensive, at least for me, for a county budget probably not.
http://www.ernstseed.com/
These are the people to ask!
It's where I got my seed. They are a good honest family business that KNOWS it's native plants. I bet if you chat them up, maybe order some seeds for the first mile into town they would tell you exactly how to encourage wildflowers on the other ### miles of road in the county
In thier catalog they also suggest periodic burning of wildflower meadows to eliminate woody species and burn off annual "weed" seeds. I will be burning mine this spring

but for county roads you would probably want to do it in the fall to discourage wildlife from gathering on the roadsides and to increase visibility around turns. After that they would just mow them once a year in the fall and burn periodically to discourage shrubs and weeds.
I think if you just let what was there grow, you would end up with mostly "weeds". Stuff blooming at different times throughout the year, annuals, shrubs. I think it would look great and save a lot of gas but
society
disagrees.