This comment is important in how it shows biking is totally different in The Netherlands. The first type of bike most non-Dutch think of when they hear 'bicycle', where the rider sits hunched forward poised to race is not even considered a regular bike here. If you go faster than 15mph you are basically considered to be comparable to a moped and doing it without a helmet is frowned upon even here.

When you are not in The Netherlands you should most definitely wear a helmet no matter what kind of bike. The fact that you can ride a bike without a helmet in The Netherlands is a sort of democratic miracle of the 70's where the public convinced the politicians to make bikes a first class mode of transport. The effect of this is that almost every road in The Netherlands has some sort of provision for bicycles, or has at least been considered to have some. And in most cases not just in the form of a line but actual infrastructures, curbs, asphalt, specifically constructed in a way to make bicycling safe.

And even when there would be no infrastructure for bicyclists, every motorist is trained to deal with cyclists. From the driving lessons (usually 40-50 hours are necessary to pass the exam these days) where a considerable amount of time is spent on being aware of cyclists, to the fact that when you're on a Dutch road and you want to turn right you are almost guaranteed to wait for cyclists you have to yield to, so not having a shoulder check habit wouldn't get you 100m out of your driveway without an accident.