I tossed around a few ideas for the theme of the diorama, but since I wanted the focal point to be the two cars, the background needed to be just that: background. Since the timeframe is around 1915, I opted for some shops and a street section reminiscent of that time period.
Initially I was planning for the middle section to be a narrow alley, and have one car impact the other from that angle, but it was too crowded and the alley would be unrealistically narrow.
I used air-drying clay for the cobbles and rolled them out for a first pass:
But thinking on it, I didn't like the narrow alley idea. Plus, shooting photos of something with a huge gap like that means you have to neutralize the background somehow. So I opted for a short building section in the middle.
After letting the street layer dry, I started the first layer of many of colors: first, flat gray.
Then some contrast:
Next: working on the buildings