Years ago, my daughters and I were driving to Kansas and we were somewhere in the pan handle of Texas and we had a similar incident. My eldest daughter was 9 and told me to stop, because I was killing the butterflies. It was incredible to see. They were yellow, not sure what they were. It was the end of May when it happened.
When I was very young in the late 1950's, we still lived in Kansas and we were on out way to my grandparents' house in Goddard, just outside of Wichita, and we had to stop for a herd of turtles crossing the road. My dad stopped the car because at first look it appeared the road was moving, then he realized what it was so we stopped and watched them. It never happened to us again.