Here is an example of a multi material terrain. The advantages of using mulit-material over multi texture is that you can have separate specular and normal maps.
I was surprised how simple this was to make.
Here are the steps I did.
Make 2 or more materials.(I made 3)
In one of the material make a texture block for your alpha mix map(s).
Make a material for the node system to assemble the terrain in.
take a look at my blender file at the node system. It is self explanatory, First I mixed the dirt and grass with the first mix-map texture, then from the mix node I attached another mix node for the road, and the road alpha map.
The geometry nodes are for the textures, so they get the proper UV map from the mesh they are applied to.
To add another material just copy exactly what i did for the road mix texture and road material.
In my example the grass is the only material with the normal map, and the specular map is turned up really high.
Just run the game engine to see the bump effect on the plane.
On a real terrain, I would tile the textures in the mapto tab in the material settings to get them to scale.
Here is the blender file:
http://www.badongo.com/file/11967418