yes, I believe that timspfd is correct.

For our Residential Road packages at Cornucopia3d, we both purchased commercial texture bases and hand-built built asphault textures that include random tire marks (quite a lot of those if you look at a real road surface), various oil spotches, small cracks and and all kinds of things. And don't forget that the painted white and yellow strips also must be dirty in a way that matches the plain asphault. Takes a semi-significant effort in Photoshop and Texturemaker to get it right, but it can be done by anyone with enough patience and some photo references of real roads.  

Use of photos doesn't quite work because you need the ability to create a very good seamless texture (or set of textures). This turns out to be quite difficult working with a photo, but much easier if you build your own in a good image program.

It is worth doing, BTW. Those super-clean apshault surfaces you see on some road models just don't look realistic, do they?