Hey Stanley
I'm not sure how you are going about making your roadway, but a method I use is to bulk it out with tinted plaster of paris, which is left to dry completely. The final layer (especially on a road this flat) would be about 5mm thick and made from a wood putty or similar, something which will hold detail well and not cure as 'harshly' as the PoP. Roughen the surface so there are no smoothed areas (an old paint brush carefully stippled over the entire surface would work) Have your AFV tracks removed and ready to drape into this final layer as seen fit, and press gently and avoid too much movement. You can treat the tracks with cooking spray before prssing into the roadway, to help with easier removal.
For this expanse of scene I'd go a cheaper option of some artists pastels. The local art supplier should have a display of all the colour range. Simply a matter of selecting a couple or three that appear the same shade as the road above. Taking a #11 blade or even a fine grit sand paper, carefully scrape the pastel into a palette or similar to obtain a good collection of dust. this can be picked up with a soft wetted brush and applied to the road directly. If the roadway is fully hardened, you can be a little brutal with the brush to make certain you grind it into the nooks and crannies, a section at a time. Allow to dry to see the resultant shade, and after this process you can add some 'European dust' in the same way, maybe adding a mist of PVA glue added to water and sprayed to the road surface prior. With some experimentation you should get a good variety of shading and find something suitable to your needs...
Good luck
Brad