From a Quora answer I wrote in response to this question:
I spent two months of the summer of 2012 cycling around Europe and thought about this a lot. Here’s what I have:
(1) Urban Layout. European cities were designed before the advent of the automobile, so they all are arranged around a central point, usually the church or cathedral. That means narrower streets that are often unsuitable for cars, more pedestrian zones, and more people living within cycling distance of a city center.
(2) Taxation Of Gasoline. There are a lot more taxes on gasoline in Europe, so that the cost per unit (liter, gallon) is usually roughly double there. So there’s a lot of incentive to use non-gasoline transportation.
I think those are the two biggest primary drivers. But then once people start riding bikes for those fundamental reasons, it starts positive feedback loops which further encourages ridership:
(3) Infrastructure: Bike Trails And Paths, and Traffic Calming. Most European countries, but particularly the Netherlands, Germany, and Denmark, have built lots of bike paths. They also do something called “traffic calming“, which is a range of laws and structures in bike-friendly areas that still allows cars through but makes it difficult or against the law to drive quickly. Finally, they frequently make modifications to intersections to make them safer for bicyclists to cross. Germans have cycle-specific stoplights. Danes have those too, and also paint the pavement red in hazardous bike crossings. There is a good list of German, Dutch, and Danish bike infrastructure improvements on page 512 of this paper by Pucher and Buehler (link to pdf — also lots of good information in general on this topic in that paper).
(4) Culture: More People Riding, Fewer Helmets. In the United States, the average person considers bicycling an athletic endeavor, not unlike swimming or boxing. In Europe, it is thought of more like a faster form of walking. Helmets are rare, making cycling seem less like a risky business (and it probably is, because of the additional infrastructure). This NYT piece has an interesting discussion of why most European countries deliberately do not make a helmet a legal requirement to ride. It suggests that at a public-health-analysis level, the health benefits of many more people riding bikes (fewer heart attacks, etc.) outweighs the detriment of more head injuries. Women in particular don’t like taking physical risks, and both the appearance and the actuality of bicycling being a “normal” activity encourages them to ride. Pucher and Buehler point out that women in Denmark, Germany, and the Netherlands make about 50% of trips (45, 49, 55 respectively) whereas that number is 24% for the USA.
As an American that sounds harsh (and I’m still wearing a helmet when I ride), but once you see flocks of helmet-less women and old people riding around Copenhagen it starts to make sense. As the safety officer at the European Cyclists’ Federation says in the article, “If you say [bicycling] is wonderful, but you have to wear armor, [ordinary people] won’t. These are normal human beings, not urban warriors.” The helmet-less-ness encourages more riders, which creates more infrastructure and more awareness of bicycles. It also makes bike sharing programs easier to implement, which also increases ridership.