The Great West Road was originally part of the Royal Road linking Gondor to the North-kingdom of Arnor. It ran between Minas Tirith and the Fords of Isen in the Gap of Rohan, where it joined the North-South Road to Arnor. The Great West Road was approximately 425 miles long.
At its far eastern end, the Great West Road met the North-way which came from Minas Tirith through Forannest, the North-gate of the Rammas Echor. The Great West Road then curved around Amon Dîn and ran westward through Anorien. The Road skirted the edge of the Drúadan Forest and passed the Beacon-hills of Gondor at the foot of the White Mountains.
The Great West Road ran through the edge of the Firien wood to avoid the marshy land north of the forest. A path marked by a standing stone branched southward off the Road to Halifirien, the last of the Beacon-hills. At the western edge of the Firien Wood, the Great West Road crossed a bridge over the Mering Stream, which was the border between Gondor and Rohan.
In Rohan, the Great West Road passed through the Fenmarch and the regions of the Eastfold and the Folde. The Road traversed the Snowbourn by way of a ford near Edoras, and a track branched off to the city. Further on, the Deeping-road branched off into the Deeping Coomb where Helm's Deep was located.
The Great West Road then curved northwestward for a ways. About two miles before the Fords of Isen there was a sharp westward turn and the Road ran straight down to the riverbank. On the other side of the Isen, the North-South Road began. A branch road led north to Isengard.
In the early days of Gondor, the Great West Road and the North-South Road formed a single Royal Road between the two Kingdoms of the Dunedain. At that time, the region between the Mering Stream and the Fords of Isen was a province of Gondor called Calenardhon. In TA 2510, that land became Rohan and the Rohirrim were responsible for maintaining the stretch of the Great West Road that ran through their land.
During the War of the Ring, an army from Mordor crossed the Anduin into Anorien and blocked the Great West Road. They dug trenches and erected stakes in the roadway with the purpose of preventing the Rohirrim from coming to Minas Tirith. But Ghan-buri-Ghan led the Rohirrim by a forgotten way through the Stonewain Valley and avoided the enemy forces. The Rohirrim arrived at dawn on March 15, TA 3019, and helped win the Battle of the Pelennor Fields.
Names and Etymology Also referred to as the West Road as well as the horse-road and the Horse-men's road.
The North-South Road and the Great West Road were collectively called the Royal Road or the Great Road. (Note that the Great East Road was also referred to as the Great Road.)
Sources: The Two Towers: "The King of the Golden Hall," p. 111; "Helm's Deep," p. 131, 133; "The Road to Isengard," p. 154-56; "The Palantir," p. 202, 205 The Return of the King: "The Muster of Rohan," p. 74-75, 78; "The Siege of Gondor," p. 95; "The Ride of the Rohirrim," passim Unfinished Tales: "Cirion and Eorl," p. 300-302, 306, 314 note 32; "The Battles of the Fords of Isen," p. 358, 363, 369-70 The Lord of the Rings: A Reader's Companion by Wayne G. Hammond and Christina Scull: "The Steward and the King," p. 635
- The Atlas of Middle-earth pgs. 53 & 88-9