Mr. Answer Person relied on help from Kirk J. Larsen, Ph.D.,
Entomologist and Professor of Biology at Luther for help. His response:
The yellow butterflies swarming on the roadways right now (and getting plastered on our windshields and radiators) are primarily clouded sulphur butterflies, scientific name Colias philodice. This common species of butterfly is a resident of northeast Iowa and is not undergoing a migration. In addition to feeding on nectar from many types of flowers, the adult butterflies are known to congregate at mud puddles and moist sand from heavy dew to feed on salts that accumulate from repeated evaporation at this time of year. The adult butterflies are mating and laying eggs that will hatch into caterpillars yet this fall that will then hibernate this winter. The caterpillars feed on common crops such as clover and alfalfa. It's hard to know if there are more clouded sulphur butterflies this year, but given the excellent growing conditions for plants throughout the summer, the butterflies might very well be responding to the warm moist weather with greater population growth.
Larsen even took these pictures of their "puddling" behavior!