Road Editor

This is the Decal road editor. Decal roads are used in every official and most of the modded maps.

How it works

To create a new road, select the road with the plus on the left and click where you want the street to start. The decal road editor places a road texture on the map, that means it will have the material the terrain underneath it has. If there are bumps or the terrain is not flat, the decal road won't flatten it, so a lot of patience is required if you want the roads to be perfectly smooth. This can sometimes be a pain with the editor tools, here's a good tutorial how to already add roads to the heightmap so you won't have to flatten them later: http://www.beamng.com/threads/tutorial-adding-heightmap-roads-using-blender.16356/

If you hate Blender, you can always fiddle around with the smoothing and flattening tools to make a smooth road surface.

Additionally, for paved roads, you need to paint the terrain with Asphalt or Concrete underneath for realistic driving physics on that road. For unpaved roads, RockyDirt is strongly recommended. It ensures realistic offroad physics, including dust kicking up from your tires and handling.