After writing this post ( http://maldotex.blogspot.com.es/2012/05/new-cracked-asphalt.html ) I was thinking a way to avoid the limitations. Finally I've found a way. First of all, we create a road material with a 1024x1024 closelook texture with 8x8 tiling. Then we create a second road material with the long range texture as diffuse (another 1024x1024), bump map from this texture and using as bump detail layer the bumpmap from the first material texture. We set 8x8 bumpdetail tiling, and 50% level of transparency. Then, we must place 2 identical roads with the two materials in the same place with higher sort priority in the second road material. That's all. It's a double win because we are avoiding diffuse detail limitation explained in the previous post, and more important, we have simulated diffuse detail plus bumpmap detail at once.
The road in the right is an example. Is a test, don't worry about the join between the road in the left. This road uses 2 1024k textures, so it's half the memory used for a 2Kx2K texture. The 4Kx4K detailed asphalt showed in the previous post was 11 MB.
2Kx2K texture -> 2.7 MB (only diffuse)
4Kx4K texture -> 11 MB (only diffuse)
This new detailed double-material road -> 1.3 MB (only diffuse)