Hm, interesting, searching "lake ponchartrain causeway curvature" (without quotes) gets 8/10 results being round-Earth stuff, but no-one bothers to prove the causeway curves, it's mostly proving the Earth is round with the power lines near the causeway, calculating the radius of the Earth using the causeway in the distance, debunking a meme image similar to this that turned out to have actually photoshopped the curvature of the horizon out of the original image...

But no proof that the causeway actually curves with the horizon! I guess that must mean it's actually flat, because there's no way you just failed to notice a third of a percent curvature in something that looks straight /s