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The Vista Ridge Tunnels are highway tunnels through the Tualatin Mountains ("West Hills") of Portland, Oregon, USA. Located in the Goose Hollow neighborhood, the tunnels pass through a hillside locally known as Vista Ridge which is a half mile (1 km) west of downtown Portland. Sunset Highway, also known as U.S. Route 26, is carried through the tunnels, three lanes in each direction. They are Oregon's busiest tunnels. The eastbound tunnel is 1001.0 feet (305 m) in length; the westbound tunnel is 949 feet (289 m). Both have 41 ft (12.5 m) of horizontal and 15.58 ft (4.75 m) of vertical clearance. The eastbound tunnel was completed in 1969, the westbound a year later.〔 (【引用サイトリンク】 Vista Ridge Tunnel (#9103+047+07353) ) 〕〔 〕 There is a six-percent grade through the tunnels. Most of the tunnels' lengths are straight, though they curve southward at the west ends 35°.〔Determined by rotating (satellite image ) with Photoshop.〕 The tunnels were built with ventilation shafts which were never used. Instead, the shafts were later adapted for electrical wiring, so as to improve tunnel illumination without marring the tunnels' appearance with visible conduit.〔 〕 To improve tunnel safety for motorists, and decrease the tunnel lighting requirements, the original tunnel entrance faces were sandblasted to remove white paint and repainted a less luminescent tan color to reduce the range of visual light adaptation required by drivers. At one time there were computer-based electronic light controls, but they were replaced by relatively simple photo detectors and relays for durability and simplicity. The night lighting level is enabled permanently and is supplemented by two or three levels of daytime lighting.〔 Except directly over the tunnels, the hillsides are steep and undeveloped forest, with some residential development along the top of the ridges. Landslides occasionally occur, but are usually minor and quickly cleared.〔 〕 The tunnels have been closed to hazardous material transport since November 1, 1994.〔 〕 As a result, US 26 is closed to hazardous material transport between I-405 and Oregon Route 217. The tunnels are located nearly at the bottom of a hill where the road gradient averages 6.5% over 2.25 mi (3.6 km). Close to the east portals is a 130 foot (40 m) tunnel underpass carrying SW 18th Avenue. Just outside the west portals is a 650 foot (200 m) tunnel under the lanes which carries eastbound Canyon Road〔 Before the Sunset Highway was built, the two lane Canyon Road extended into Goose Hollow. Street signs near the Vista Bridge indicate the junction of SW Jefferson Street and SW Canyon Road. 〕 from the exit ramp into Goose Hollow. The Robertson Tunnel for MAX Light Rail is underground approximately 800 feet (250 m) to the west. == Traffic volume == They are the busiest tunnels in Oregon,〔 〕 12% busier than I-5's 6-lane Interstate Bridge, and 2% busier than I-205's 8+ lane Glenn L. Jackson Memorial Bridge.〔Glenn L. Jackson Memorial Bridge carries 137,000 average daily traffic; the Interstate Bridge, 124,500. Obtained 2004 values from 〕 They carry about 25–35 percent more traffic than New York's Lincoln Tunnel〔 Lincoln Tunnel's average daily traffic is consistently around 100,000. 〕 which claims to be one of the busiest tunnels in the world.〔 (【引用サイトリンク】 The Helix Fix – The Lincoln Tunnel Helix Rehabilitation Project ) 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vista Ridge Tunnels」の詳細全文を読む スポンサード リンク
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