As a dutch biker I would only wear a helmet when mountain-biking. The rest of the roads are very flat, we don't have much inclines. And our bikes are mostly in good condition, good brakes etc. In the cities people tend to have worse bikes because of theft but speed is lower there, so less danger. And last but not least, we have bike lanes, so you just wont come across cars regularly, a car has to ignore red lights to come in contact with bikes, so accidents are not that common.