Art....
The arches that support my city are made from pink insulation foam sheets. It was as simple as drawing out the design directly on the foam and then cutting it. From there you can paint and weather to your taste and then use a soft lead pencil to draw a block or brick pattern. The pencil creates the mortar joints and a relief to the surface. The foam can be bent into graceful curves, etc..... The picture below, even though it is not in a bridge form as you intend, shows the finished arches for the city....
Alan
