The Forest Road was the road that ran down from the High Pass, crossed the River Anduin and then plunged among the trees of Greenwood the Great (later called Mirkwood). It followed a nearly straight course through the central parts of the forest, somewhat to the south of the Mountains of Mirkwood, and emerged on the banks of the River Running. In ancient times the road was actually extended all the way up to the Iron Hills, but after a while the road fell into disrepair, and the Dwarves of the Iron Hills lost contact with the capital of Khazad-dûm.
The original route planned for the Quest of Erebor was to cross Mirkwood using the Forest Road. However, the Dwarves had been dwelling in the Blue Mountains in the west for many decades and were not well informed of recent developments in the area. When the company arrived at the house of Beorn, however, he informed them that the eastern portions of the road had fallen into such bad disrepair that the route was impassible. Instead, Beorn advised them of a secret path made by the Elves of Mirkwood that ran across northern Mirkwood.