In 1955, Afghanistan and the former Soviet Union signed an agreement to initiate joint development of the Salang road, initially via the historic Salang Pass route. The tunnel was opened in 1964 and provided a year-round connection from the northern parts of the country to Kabul. The tunnel was the highest road tunnel in the world until 1973, when the United States built the Eisenhower Memorial Tunnel — just slightly higher and slightly longer — in Colorado in the Rocky Mountains.