In places where there are lots of bikes, most people who are riding them are making short journeys purely for transport. For a short ride of a few kilometers, it really doesn't matter what you're wearing. Any clothes that are comfortable to wear in general will be comfortable to wear on a bike for fifteen minutes.

People who are riding longer distances, pushing harder and aiming for fitness rather than just getting from A to B will start to consider clothing that's more oriented towards exercising and people who are going even farther and pushing even harder will start to wear cycling-specific clothing.

I see this exact thing when I cycle at home in Oxford. If I cycle directly into town, most of the people I see on bikes are just wearing whatever clothes will be appropriate at their destination. If I take a more circuitous route around the outskirts of town before heading into the centre through an area where not many people live, I see a lot of people wearing "general exercise" clothing. If I head away from town, out into the countryside, most of the cyclists I see are wearing clothing designed specifically for cycling.

Local here, have lived in Amsterdam for just over a year and a half, through the cold wet winter. I'm not a Dutchie, but have embraced the fiets lifestyle here.

Let me just say that the jeans you get in Amsterdam are the same as what you would buy anywhere else. I ride in Levis 511 jeans every day and have no problems. People here more or less ride in all sorts of clothing (that includes suits, dresses and high heels). You never really go that fast here in the city and you're always kind of stopping and starting, so regular clothes are fine.

When it rains (which happens a lot) most people either catch public transport or wear rainproof gear over their regular clothes. Some even use storm umbrellas. The philosophy here amongst the Dutchies is that there's no bad weather only bad clothing.

The bikes themselves are set up a bit differently here too:
- Every bike, almost without exception has mud guards or "fenders"
- The chain is almost always sealed inside a chainguard to avoid it getting too dirty, as well as the rider
- The vast majority of bikes are upright riding position, as you don't really tend to go that fast in Amsterdam with the cobblestones and narrow streets

In terms of type of bike, lightweight performance bikes don't last very long here and are not practical. What you mostly see are Omafiets, Papafiets, Mamafiets, Bakfiets and various heavy duty city commuter bikes. Powered bikes are also very popular here and often used to cut down travel times for longer distances.

Bikes here tend to be heavy duty so they don't get smashed up at the bike racks, low maintenance as they usually sit outside all the time and focused on comfort as it's not really an "activity" you plan for with specialised clothing but a primary way to get around, with most journeys not being longer than 20 minutes or so.

Outside of the cities you'll see plenty of lycra clad cyclists on their lightweight performance bikes cycling more for recreation on weekends etc.

Jeans aren't as bad as you think. I think the biggest factors are short journeys, mild climate, and being used to it. With efficient infrastructure this can account for a lot of commuting. Apart from the efficient infrastructure that covers most bike commuting in the UK as well, and most people here don't get changed.

I sometimes don't get the chance to get changed and have to commute in normal jeans for up to about 30 minutes, even an hour in the past (up to about 15km in traffic). In the UK we have a similar climate to the Netherlands. In the dry, and if its not too hot, it's not uncomfortable, on a hybrid or tourer. On the latter I'm not generally going flat out, or using the drops, because of the traffic.

Riding in wet jeans for more than a few minutes does start to rub, and can get cold when you stop. I get too hot for overtrousers, which is a further reason I prefer to get changed, but I also don't like to hang around. Keeping spare clothes in work deals with getting caught unawares. I don't recommend emulating my colleague who used a heat gun to dry her jeans while wearing them, though the one in my lab goes quite cool.

The main difference in bike design is that Dutch bikes have enough of a chain guard that you're not going to catch your jeans in the chain or get them dirty. Ankle straps/clips or simply tucking your trousers into your socks deals with this.

There's probably a form of selection bias going on - people who think and talk about what to wear on the bike are likely to have specialist clothing and choose to wear it. They're also likely to ride far enough that it's worth doing so.

You are surely aware that there is an industry out there that tries to sell you things you only marginally 'need' - special clothing for any sport is not unaffected by that.

There is certainly a gain by wearing special cloth (and shoes) while biking, but unless you are seriously exercising or training for a competition, the difference is marginal.

Of course, in the respective forums you will find lots of people that are convinced otherwise, and will tell you (and themselves) it makes a world of difference.

Just try yourself with an open mind.

As someone who biked daily for transportation and recreation for years, I can tell you it doesn't take any miracle to do it in street clothes.

David Richerby already mentioned that most practical bike trips are short. Mine were usually 10-20 minutes. But many times I've gone on recreational rides of 2-4 hours. And I've never owned any special "biking clothes". The most I ever thought about clothing was when my jeans started getting worn out from occasional encounters with the gears!

It probably depends on the person, but many feel perfectly fine in street clothes.

I've never been to Amsterdam, but I do remember reading a comment by someone from there once about what it was like. He said that people there tend to ride at a more leisurely pace, and everyone from all walks of life ride bikes. It's just ordinary people riding around to get to places.

In contrast, in non-cycle-friendly cities, cyclists ride hard to keep up with the traffic, and since there is some danger, it's mostly the more hardcore athletic types who ride.

If you are riding at a moderate pace and the weather is cool enough, then riding in jeans is fine.

It's just working. I do have proper biking pants, but I found that especially on more rough terrain (MTB, Enduro, Trail...) I often simply wear my Levis 501, both uphill and downhill, to avoid problems with twigs slashing either the trousers or my skin. No problems, so far, it feels perfectly fine.

One of the more significant differences between a dutch city bike and a performance-focused road bike is the saddle.

Saddles for transport tend to be larger and softer. That way not having padding isn't nearly as bad because the padding is built into the saddle.

The chain is always protected by a guard to avoid loose pants getting caught in it and mangled.

Fenders and mudflaps are also a constant fixture to deal with rain.

On women's bikes there will sometimes be a skirt guard to protect said garment.

The tires are on the chunkier side with lower pressure. This makes for a much more comfortable ride in general.

I will take the liberty to interpret the question slightly more broadly, on how bicycles are adapted for riding in ordinary clothes. Apart from raingear, I've never changed clothes for cycling in my life (not when I went on a 700 km bike tour either).

Skirt guard / coat guard

You don't really need fenders for cycling with jeans. All fenders do is stop you from getting wet and dirty, so you should want them with any type of clothing. However, people not only ride bicycles with jeans, but also with skirts (starting yearly on rokjesdag). With a skirt, dress, or long (over)coat, a skirt guard/coat guard prevents clothes from ending up in the spokes, which not only damages the clothes but may well lead to accidents.

SkirtCoat guard. Source: Wikimedia Commons

If you are commuting by bicycle and commuting in your normal clothes, as you should, and your normal clothes include anything that may get stuck in the spokes, a skirt guard / coat guard is recommended. As pointed out in the comments, the guard on the bicycle in the photo is unlikely to be intended for protection of long dresses, as a traditional "ladies bicycle" would have a lowered top tube to assist with the same.

There are sometimes heavy rains but most often it is only drizzling. Furthermore, there is no such thing as a whole day rain. It can rain several times half an hour per day and in between your cloth can dry out. Last but not least you can sit high (your foot is almost straight when pedalling) and you hold the handle by leaning forward (cover).

If it is heavy rain then even the Dutch bike in additional raincoats and trousers or with umbrellas or they do not bike.