This is most likely due to the way POM works. At more and more glancing angles, you can see the steps for each ray. It’s just a limitation of the technique. The way to get around this is to increase the number of steps, but there will always be an upper limit.
Ryan talks about it in this stream starting at 13:02:
You can try to increase the number of steps POM uses as the camera is at a more glancing angle (Camera Vector Dot Normal), so you don’t pay that cost when looking down from the top. For something as large as a road, you probably also want the steps to fade off to nothing after a certain distance from the camera.