@Mark Boyce Mark, as it turns out making the clay streets is not all that bothersome. The brick streets are easy to roll out and emboss. The key is to use a "modified" air hardening clay such as NARA or DAS. I can get NARA at HobbyLobby usually on sale for 50% off basically $2 per pack. I'll roll the clay out on a modelling mat and trim to have straight sides. Then let it dry for a day or two. 2 mils seems to be a decent thickness. It will shrink a little and can get wavy if it dries too fast from one side than the other. The clay is rolled out to 12 to 18 inches long and 4 inches wide (the width of the roller). Paint like any other plastic form. Once dry I trim with a pair of scissors or hobby knife. Way cheaper than buying the plastic brick sheets. Bricks are roughly 2mmx8mm. Typical building bricks are 1.5mmx4mm. So for streets the bricks look fine, they may be a touch too big for buildings.
NOTE: I tried using natural unmodified clays, but found they shrink a lot and have little strength in thin sheets. I have searched for the chemistry behind the so called "polymer" or "modified" clays and had moderate success in getting specific details. NARA is supposedly made from corn starch, fibers and water. Not really a clay.
