They bloom profusely if there's been enough rain and this year we've had plenty, so there were a lot of them. In the fall, the plant turns yellow and really shows up against the darker orange and tan grass of fall and winter.
Hint: The cultivated variety is an annual that is one of the most ubiquitous container plants in the midwest.
Answer below the fold
Answer: Wild Petunia [ruellia pedunculata]
According to Audubon, the plant likes rich woods, especially limestone sites. We've got the limestone and woods, but these babies mostly grow on barren clay and the road edge. Interesting.