The transport ministry on Tuesday released guidelines related to the installation of remote-controlled traffic barriers at underpasses that are at a high risk of flooding, to restrict access during heavy rain. The move comes on the heels of a series of deaths at flooded underpasses, including during heavy rain in Chiba Prefecture in August. According to the ministry, there were 3,841 underpasses across Japan as of the end of March 2025.
Under the guidelines, compiled for road administrators including local governments, underpasses around the country will be categorized into four levels based on flooding risk, and administrators will determine the classification of each underpass. The guidelines call for the installation of barriers at underpasses deemed to be at particularly high risk, especially those where flooding has led to casualties or property damage in the past. Traffic will be restricted in advance when weather alerts corresponding to Level 4 emergency warning are issued or when hourly rainfall has reached or is expected to reach a certain level.
The guidelines also stipulate that traffic restrictions and measures to prevent vehicle entry must be put in place when drainage facility abnormalities are detected. Meanwhile, the Japan Automobile Federation (JAF), an organization that provides roadside assistance, called on drivers to turn back immediately if they see flooded roads. After Chiba Prefecture was hit by torrential rain on Aug. 13 and 14, JAF received 4,442 rescue requests in the week up to Aug. 19 as streets were flooded in various parts of the prefecture.
In 2024, JAF conducted experiments on a 100-meter-long water-covered road using electric vehicles, hybrid cars and gasoline cars and found that when the cars entered the road with water 30 centimeters deep at 30 kph, all of them had water enter the engine compartment, interfering with driving. Road conditions can suddenly change during heavy rains, especially at underpasses, making it difficult for drivers to assess the depth of water over streets, it said, urging drivers not to underestimate the risk of getting stuck.
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