Loudly and visibly changing the rules doesn't "create offenders". Offenders aren't victims of changed rules.

It has been shown time and again that lowering speed limits in cities reduces traffic accidents and emissions at close to no costs to the flow of traffic.

My own city (in Germany, so it really was a heavily-criticized decision) lowered the speed limit on one of the major arterial roads to 30 kph. It is one I have to use regularly, and oh boy, let me tell you: I was soooo opposed to the change. Yet, it really only changed how fast you arrive at the next red light. There is literally no discernable change in how long it takes to pass that street, especially during rush hour. Traffic just got a little more fluid.

It is, however, the street with the most speeding tickets in town. I regularly see one or two mobile speed cameras along the way. And I've never been fined. You got to wonder...

Wait school zone speed limits in the US is higher than 30km/h ?

In my country school zone speed limit is usually 20km/h and there are tons of residential areas as well as more and more cities that restricts big portion of their roads to 30km/h.

This is mostly done to reduce noise pollution as well as mortal accidents.

Thankfully Amsterdam isn't the first city to do this and most of it is also limited to 30 kph anyway, so we actually this will work out fine.

I can ride faster that 30 km/h on my bike. I can continually do that with as little as a 5% decline from the horizontal axis.

5% is pretty steep, I wouldn't be surprised if one could reach 30 kph without pedaling at all.

Also 30km/h with a 9kg bike isn't 30km/h with a 1600kg toyota when you hit someone

If you want to go fast go around the city

1. Amsterdam isn't the kind of place where you'd be doing 30km/h. In car or on bike.

2. Amsterdam is one big highly populated area. See point 1.

3. If you do try going over 30km/h in Amsterdam, you're frankly a danger to yourself in most places on bike and a danger to pretty much everyone else when you're in a car.

Many major European cities already implemented this, and I've yet to hear of a single one where it turned out to be "a big mistake". Can anybody from these cities report (e. g. Helsinki or others)?

but going below US school speed limits is going to create lots of offenders

That's not really relevant in Europe.

You obviously never been to Amsterdam

30 km/h is somewhere around the point where pedestrians can walk away from a collision with a car. Not unharmed but likely without broken bones.

Yes you can bike faster, but a bike has much less mass so they're less dangerous for pedestrians in collisions.

The limit before was 50km/h. The city is just too crowded. By lowering the speed limit even more you force people to take the highways if they want to reach the other side of the city.

Actually there's so much road crossing from people and bikes in Amsterdam that cars kind of already go to 30km/h in most streets