For more detail this asset from the Unity Asset Store: Click Here
This is a paid asset, but now you can download Road Builder for FREE, Please keep in mind this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Road Builder v2.3
Road Builder Lets you create non-linear roads easily by using a network of nodes. With easy to use buttons to extrude, join, insert nodes and create mesh. Building new roads has never been so easy.
The options let you create one large mesh, or smaller meshes to help with culling. Drop to ground, and whether to add collision or not. And you can control the intersection of the road, to set the height and width of the curb.
This package has all you need to create 7 different road types from normal (with and with-out pavement), grass, sandy, race track and even a rail track. And you can add more road textures/materials as you need. You don’t need any programming skills to create amazing looking roads.
Version 1.1: Supports Unity 5.5. Lets you change the width of the road per node.
Version 1.2: Lets you change the materials used on the road per node.
Version 1.3: You can now place the road nodes closer then before.
Version 1.4: Improved UV on crossroads and junctions. Added Rounded inner corners option to crossroads and junctions.
Version 1.5: Update the mesh generation to work with BakedLights.
Version 2.0: Added terrain modifier.
Version 2.1: Nodes can be placed closer to the Junctions and cross roads. Junctions can now use 5 or 6 connecting roads.
Version 2.2: Only rebuild the mesh for part of the road that has been changed. AutoBuild Option that will create the road each time a node is move or updated.
Version 2.3: Updated meta tags and category
Version 2.4: Fixed a bug where the Terrain modify would fail if the road was outside the terrain. And added drop to ground options on any selected nodes
Version 2.5: Update the UV mapping near junctions and crossroads to stop texture squishing
Version 2.6: Added the option to render the under side of the road.
Example and documentation can be found Here
