It is not unreasonableness that cyclists (here at the traffic lights on the Berlagebrug over the Amstel in the east of the city of Amsterdam) are all riding without helmets. It is common sense. That of the people in cars: they respect those on bicycles, there's a cooperative relationship. Probably because those in cars are often enough also those on bikes. And, of course, the city (the municipality, whoever is responsible) also does something for a prosperous coexistence. Cycle paths everywhere, parking garages for bicycles. And a sophisticated network of paths.
As good Germans with the relevant experience from the city and country, we naturally had our helmets with us on our trip to the Netherlands and used them at the beginning. That was in the countryside, on the coast - and we weren't the only ones wearing helmets. Virtually all the racers had one on, and there were a lot of them! So far, so good, so right. The not-so-fast ones - to which we clearly belong! - all managed without helmets, because even in the countryside there is, firstly, an almost complete network of cycle paths and, secondly, considerate motorists, apart perhaps from newly arrived tourists without a Dutch bike feeling.
