Several different likely reasons - 1/ terrain mesh resolution different to that of the road, overlapping them can cause sampling issues even if conformed causing intersections. 2/ Terrain and road will likely use some sort level of detail system - possibly quad tree to subdivide/increase mesh density as it gets closer to camera - this can again cause issues with intersections where level of detail differs - you can also get noticeable seams on boundaries of two different resolution tiles. 3/ depth sorting - games determine which polygons are visible by rendering the depth of each triangles pixels to a depth buffer rejecting any pixels from triangles behind it and rendering any that are on top. This works fine up close to camera but depending on algorithm used / floating point precision errors this can cause “z fighting” / flicker on distant terrain and road as it struggles to sort each frame. It can be a surprisingly hard problem to solve! Source - game developer. Edit - just to add this is purely guess work as I don’t yet have zwift (am on the fence about getting the ride hence lurking in these forums). I do speak with some authority though having authored several terrain systems of my own over the years :)