I was puzzled by that abrupt change at the border as well. It could be market penetration. Maybe bike infrastructure like the bike knooppuntnetwerk plays a role as well. http://www.nederlandfietsland.nl/knooppuntroutes/overzicht-k... Although dedicated bike lanes fall of much more gradually at the border f.i. https://www.google.de/maps/@51.6623341,6.998291,9z/data=!5m1... As a Dutchman I never realised how unique this dense network of dedicated bike lanes and route signs is. Until I met some Canadian bike head expats who couldn't stop raving about it. It seems that if you're into touring on bike and getting around by bike the Low Countries are without equal.