Sorry been swamped of late!

To clarify my thinking,

1 - using procedurals to enhance your image maps - this can help obscure any pixelization in the shots (both AsileFX & GeekatPlay have tutorials that can help w/that aspect of things). 

2- as for the tiling comment, I had initially thought that since you'd defined a meterial region for the road, why not apply a single instance of the road image and tile it along the -X- axis - feeling that using a smaller but 300DPI image and tiling (w/interpolation of course)  would have helped a lot plus adding the function driven mix between the two should have completed the look you were after. Sadly taking more than a moment to think this one thorugh, image maps for the overal road surface might not be the best approach. Perhaps discreet material zones for each color/material on the road surface, then you can create tiling surfaces that sould work again w/teh caveats from above about image maps.