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Ice roads (ice crossings, ice bridges) are frozen, human-made pathways on the surface of bays, rivers, lakes, or seas in polar regions. They link dry land, frozen waterways, portages and winter roads, and are usually remade each winter. Ice roads allow temporary transport to areas with no permanent road access. Seen in isolated regions of northern Canada, Alaska's Bush, northern Michigan and Wisconsin,〔http://blogs.mprnews.org/newscut/2013/02/driving_on_an_ice_road_do_you/〕 northern Scandinavia, Estonia, Northeast China and Russia, they reduce the cost of materials that otherwise would ship as expensive air freight, and they allow movement of large or heavy objects for which air freight is impractical. Ice roads differ from winter roads in that they are built primarily across frozen waterways; however, an ice road may in some cases be part of a longer winter road whose path traverses both water and land. Ice roads may be winter substitutes for summer ferry services. Ferry service and an ice crossing may operate yearly at the same time for several weeks. An alternative meaning of ice bridge is a natural ice road or a structure formed during glaciation. These were used in prehistoric migration. == Function == Because ice roads are flat, devoid of trees, rocks and other obstacles, they have a smooth driving surface. They are largely plowed across frozen lakes, with a short overland portage between the shoreline of one lake and the next. Similar to ice roads, ice runways are common in the polar regions and include the blue ice runways such as Wilkins Runway in Antarctica or lake ice runways like Doris Lake Aerodrome in the Arctic. Ice is used as an emergency landing surface. In general, these roads are built in areas where construction of year-round roads is expensive due to boggy muskeg land. When frozen in winter, these obstacles are easier to cross. Ice roads such as the stretch between Inuvik and Tuktoyaktuk, Northwest Territories, Canada provide an almost level driving surface with few detours several months of the year. Ice roads and winter roads are used where year-round roads are expensive or impractical. When frozen in winter, the waterway crossings can be built up with auger holes to flood and thicken the crossing. Clearing snow (which insulates and warms) makes ice thicker, more quickly. These seasonal links last anywhere from a few weeks to several months before they become impassable. After an ice road is plowed across a lake, the ice there gets much thicker than the surrounding lake ice, because the snow cover is swept off — exposing the road directly to subfreezing air (temperatures as low as ). When a lake thaws in the spring, the ice under the road is the last to melt, and in the summer, traces of the roads can still be seen from overhead in a bush plane, as bare strips remain on the lake floor where the ice blocked light and prevented plants and algae from growing. 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「Ice road」の詳細全文を読む スポンサード リンク
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