For many years there has been a tendency that a large amount of precipitation has fallen in a short time. Næstved and Greve are examples of two Danish towns which have experienced heavy precipitation activity. The heavy precipitation has given problems, such as water in the basements of private homes and public institutions, but the road network has also been flooded, which has made travelling difficult. In England, the motorways M5, M25 and M50 have been closed due to flooding, caused by extreme precipitation in the summer of 2007. In the worst case, M25 was closed on a warm summer day, where drivers could neither come backward nor forward. The British emergency services had to fly drinking water supplies to drivers in distress by helicopter. The last drivers could leave the M25 after being caught for twelve hours. The operation and help cost large amounts of money.

The climatic changes which are predicted by the Intergovernmental Panel on Climate Change (IPCC) point towards more precipitation seen on average throughout the year as well as stronger and more intense shower activity in summer. This means that the drainage systems of the road network must be prepared to drain larger amounts of water and in extreme cases in relatively short periods of time. The existing drainage systems are designed and constructed on the basis of previous precipitation events (statistics). Similarly, more net precipitation leads to larger water stream flows, possibly causing problems with present culverts that are designed based on outdated statistics of high flow events. It is obvious that an upgrade must take place soon. But if this is done from “one end to the other” it becomes very comprehensive and expensive and presumably also unnecessary. In many places, additional capacity already exists.