In the mountainous regions of the Tama area and Tokyo islands, areas rich in nature with steep terrain, tunnels are structures essential to facilitating smooth traffic flow. While working to preserve the natural environment, the Bureau constructs tunnels to help avoid natural hazards, including rock fall areas, as well as to eliminate curbs with poor visibility and reduce traffic congestion during peak travel seasons.
In the western Tama area, there are sections of major arterial roads that become impassable during times of extreme weather. As a measure to prevent the isolation of local communities, the Bureau is developing a bypass on the southern bank of the Akigawa and Tamagawa rivers that adopts the tunnel structure, in order to provide a high level of safety.
Of the planned 7 km of roadway to run along the southern bank of the Tamagawa River, 3 km has been completed. Construction of the Shiroyama Tunnel in Okutama-machi, Nishitama-gun is currently underway.