Under the "sustainable safety" vision in the Netherlands roads for through traffic and distributer roads should have only one function: getting large quantities of traffic from A to B in a safe manner. These roads should have no other function like parking or access to private homes and other destinations. On roads like these there is a separation of types of traffic by mass, speed and direction. So naturally bicyclists have their own separate cycle paths.
see more info: http://hembrow.blogspot.com/2012/01/campaign-for-sustainable-safety-not.html