Tracks

highway = tracks:

This tag represents roads for mostly agricultural use, forest tracks etc.; usually unpaved (unsealed) but may occasionally apply to paved tracks as well, that are suitable for two-track vehicles, such as tractors or jeeps.

If the way is not wide enough for a two-track vehicle, it should be tagged as highway=path.

https://cloud.githubusercontent.com/assets/2665840/6720029/9b764622-cd95-11e4-9b9f-de03f3b46af9.jpg