Crossings typically have either wood, steel plate, or dense rubber plates between the rails. I once experimented with printing a steel diamond tread pattern onto paper and using that between the rails. It had enough 'give' to not interfere with the wheels.
If you want asphalt between the rails, 1mm black craft foam with gray paint sponged on works well for asphalt. The stuff you find at Michael's is 2mm and probably too thick. But 1mm will work on code 100 rail without interfering with the wheel flanges. You'll probably need to order 1mm online.
I've used everything from manufactured kits which can be either plastic simulating steel plate between the rails and on the outside of the rails, to wood simulating...wood. These are available as kits for a nominal cost. Leading up to the crossing I've used road-simulating sticky paper over a bare plywood base, to plaster-of-paris painted with "asphalt" latex-based paint. In the case of the later, you could also use a light gray paint with gray ballast to simulate gravel. When planning you roads beware right-angle curves and narrow roads if you plan to have 18-wheeler truck-trailers right near the crossing, or you might wind up with the kind of disaster that happened recently on the prototype!