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| drainage gradient ： ウィキペディア英語版 |
drainage gradient
Drainage gradient (DG) is a term in road design, defined as the combined slope due to road surface cross slope (CS) and longitudinal slope (hilliness). Although the term may not be used, the concept is also used in roof design and landscape architecture.
If the DG is too low, rain and melt water drainage will be insufficient. This results in water pooling on the road surface, thereby increasing the risk for hydroplaning and resulting wet pavement vehicle crashes.
== Minimum drainage gradient ==
Most road design manuals require drainage gradient to exceed 0.5%, in order to drain water and prevent excessive skid accidents.〔("A Policy on Geometric Design of Highways and Streets" (AASHTO Green Book) )〕
One exception to the minimum 0.5% DG limit can be found in the Norwegian road design manual, where the minimum drainage gradient is 2% instead of 0.5%.〔(Håndbok 017 Veg- og Gateutforming )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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