The cause of the decline is mentioned in the article:
"- Oslo has received several regulations that have reduced the number of places where it is possible to drive. This is especially true within the city center and downtown areas. People have started cycling more. The risk of accidents between motorists and cyclists has been reduced. The more you separate the different road groups, the less the risk of serious traffic accidents. And then we see that the speed limit has been lower on several roads, says Steen."
"- Oslo has received several regulations that have reduced the number of places where it is possible to drive. This is especially true within the city center and downtown areas. People have started cycling more. The risk of accidents between motorists and cyclists has been reduced. The more you separate the different road groups, the less the risk of serious traffic accidents. And then we see that the speed limit has been lower on several roads, says Steen."