I'm a programmer, not an artist. Instead of me spending loads of time creating a realistic look, that probably won't look good and increasing the file size by 10x, you can use any material. I feel like people want to use their own materials anyways, the default ones are just simple ones to show the system. If you want realistic textures I recommend textures.one . The roads, intersections etc use normal materials so no custom shaders are used (except for the prohibited area)