I have seen many of these out in the grasslands and never really thought much of them being other than distorted trees… was not aware they were road signs
"Living history doesn’t get any more “living” than this: a post oak, likely over 150 years old and bent by Native Americans for use as a trail marker.
Steve Houser is a North Texas arborist and has documented about 100 likely trail marker trees, made by bending a sapling with a strap tied down with stones.
“I feel an obligation to do it for future generations,” Houser said. “Because once they’re gone, they’re gone.”
Just like we follow highway signs to find points of interest, marker trees often pointed to food, water, shelter, medicine, and low river crossings.
“Almost in total darkness, you would be able to pick out these shapes and it would save your life,” said Dennis Downes, a bent tree expert from Chicago. “Get you to shelter, get you to safety, eluding other tribes.”
http://www.wfaa.com/news/Bent-trees-in-North-Texas-lead-the-way-back-in-time-188431161.html