I did a road test sheet in foam. I liked the way it took paint and a wire brush gives the foam a nice looking texture. I'm not sure I like how foam "cracks". I mostly used a pencil and thought it looked quite unconvincing; Too "puffy" rather than jagged.
On the other hand I really admired the roads featured in TNT and Joe shared the link which explained how it was done. I thought they looked better than my foam test and liked the added durability the shingles offered over the foam.
I'm planning to have a go at a road surface very soon myself. I'm planning to cut MDF roadbeds then layer the shingle over top. If I tear it up and introduce potholes I'll add earthwork into the deformations and hopefully it will look ok. Before I paint them I will probably varnish the built sections because I'm not sure how well the raw shingles will play with acrylic paint.
If I prototype my plan in the next few days maybe we could post up what we have done and compare notes.