In the mid-2000s, modding San Andreas to change the map this drastically was considered impossible. Most mods edited textures or added a few cars. The Mafia Mod added a with working traffic paths, ped paths, and weather zones. It crashed constantly in early builds, but the ambition alone earned it legendary status.
Installing the GTA Mafia mod requires some technical expertise, but it's still accessible to most players. Here's a step-by-step guide: gta mod mafia