It's Oklahoma. I see people every day driving down the rural roads giving the index finger wave vs the middle finger wave you get in the big cities.
It's been interesting over the years that folks moving to OK or visiting see people typically on two lane rural roads raise the index finger from the hand on the wheel as a wave to someone driving in opposing traffic in rural areas. I've seen several letters to the editor in our local news from visitors to our state saying when they first got here, they were offended until they realized it was a greeting/hello and saw which finger was being used.
Don't see it so much in urban areas. Traffic is too hectic I guess.
Not so much the single finger, but out in the Panhandle, we tended to raise more than one from the steering wheel.
Then I read a comment from someone that was passing through Cimarron and Texas Counties that said something to the effect that "those folks probably waved because they hadn't seen anyone on the roads for quite some time. Probably waved just to acknowledge the presence of the other in such a lonely place."