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(Measure in Japan to close road when rainfall amount reaches the standard value to prevent damage to cars, even if no landslides, flashflood or falling rocks have occurred.

In Japan, where the land is mountainous and the geology is fragile, landslides, flashflood and falling rocks occur frequently during heavy rains, sometimes causing damage to road facilities and to cars and people traveling on the roads.

Basic countermeasures against such disasters on roads include removing or fixing the causative factors such as sand and rocks, and installing protective facilities. Another example is to prevent damage to cars and people by closing road (restricting traffic) when hazards are anticipated.

In Japan, the first case that triggered the system of closing road due to the threat of landslides, flashflood and falling rocks was the 1968 flashflood accident on National Route 41, which caused two buses to fall off the road. Torrential rains caused a flashflood on the eastern slope of National Route 41 in Gifu Prefecture, hitting two large buses that were passing on the road.

This caused the buses to fall into Hida River, which flows on the west side of the road, killing more than 100 people. Until this accident, it had been common practice to avoid closing road as much as possible since closing road affects local people’s daily life and regional economic activity, and actually they intended to wait until after a road had been damaged by a disaster to close it to through traffic. However, this accident changed that.

In the year following the accident, the Director General of the Road Bureau of the Ministry of Construction (now MLIT) issued a decision that the government should designate sections where damage had occurred or was likely to occur such disasters in the severe weather, and set the standard rainfall amount as criteria to be followed in order to close the road.

Specifically, when there is a correlation between the occurrence of disasters/accidents (landslides, flashflood and falling rocks) and weather conditions, set the standard rainfall amount (using continuous rainfall, hourly rainfall, or both) based on past records, and when the amount of rainfall reaches this standard, they close the road. The actual designation of sections was rapidly promoted in a period of about 10 years 2 until around 1980, and more than 200 sections were designated on roads administered by the national government throughout Japan (the number of designated sections has since decreased due to the sequential implementation of countermeasure works). In addition to these, there are also sections that would be closed when danger is anticipated through patrols and other measures