Make some pictures yourself. I found that shooting from the side (that I see wide part of road and some of the roadside) is better than creating a perspective photo. I usually do a photo, then holding the camera in the same (almost) position, move some meters to the side. This way I can make some kind of "panorama" of the road (more detail, less chances to get repetitive patterns from textures latter).
Then the crucial thing is to make seamless textures. One simple trick is to work on the photos, make one big picture of them all, then crop to desired size (e.g. 1024 x 512). All layers have to be flatten. Then you go in Photoshop to "Filter-Other-Move" (not sure exactly on English names, sorry) and set the Move to the half (in my example 512). You will see how texture repeats in the middle. Now you can paint, stamp etc. to make it better. I usually (before that "filter") make a copy of the layer and then from top (moved) layer I erase (random brush) some of the middle part. In most cases it's enough.
Sometime before the "Move" I use High-Pass Filter to help the picture to be a little more "smooth" in terms of shadows, gamma etc.
Of course you can make seamless textures in both directions, like grass or other ground.
There are several other plugins for PS that make seamless textures - like Pixplant2, but you have to pay for them.
When saving to DDS, you have to add some mip-maps (6 should be enough), then you textures will have more detail in distance. Remember always to use proper sizes - a powers of 2 (4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096...). Textures can be then squares or rectangular (they say that squares work better and take less performance from machine). Sometime you can save textures like 384x384, but mip-maps will not be available.