Although the Homer Tunnel is wide enough for a bus and a small vehicle to pass each other, traffic lights operate during the busy summer months to keep it safe. The tunnel is a fairly steep 1:10 gradient running east to west.
The tunnel passes through the Darran mountain range, below the Homer Saddle, and into the Cleddau Valley, 945 metres above sea level. Both the tunnel and saddle are named after William Henry Homer, who discovered the saddle along with George Barber in 1889 while on a surveying expedition. At the time, Homer is said to have suggested that a tunnel under the mountains would be the best way to bring a road to Milford Sound. However, it took many decades before his idea became reality.