First for clarification :: From your post am I correct in thinkning that your heightfield map is 10K in size? Are you using a color variation to 'paint' your road surface?"?

Second - in general :: try activating interpolation for your maps that have pixelizxation bilinear or bicubic (gausian qwill anihilate the details in this case so dont try that mode)

Third -- for a dif. approach :: Why not use function driven procedurals, or again a mapped procedural mixing in a tiling image - if you really want the image mapped version (edge repeat only for the map).

Keep us updated!

Regards,

AF