That isn't that different from 40 years ago. The Dutch-Belgian border has been technically open since 1944 (the BeNeLux as a customs union dates from then; see http://en.wikipedia.org/wiki/Benelux. In practice, the border will have opened a bit later, as World War II was still going on)

The Dutch-German border was closed, but in practice, outside main roads, there were only roaming border controls. The tripoint of Germany-Belgium-the Netherlands lies on a hill without main roads, so one could cross the border freely there even in the 1960's.