Ultimately the limit is mostly decided by the width of the road. You can set a 10km/h limit but if your road is wide and straight, many will just go at whichever speed they feel comfortable with. The reverse is also true: I can bet it was very rare for anyone to ever reach 60km/h on streets like those shown in the picture.

So to really enforce slower traffic, narrow down your street to the bare minimum required for a car to pass through. Add obstacles as small turns or bumps. You'll also gain space for sidewalks and cycling paths.

> In traffic, energy used over a specific period of time (power) translates into speed. In this case, the critical quantum will appear as a speed limit. Wherever this limit has been passed, the basic pattern of social degradation by high energy quanta has emerged. Once some public utility went faster than 15 mph, equity declined and the scarcity of both time and space increased. Motorized transportation monopolized traffic and blocked self-powered transit.

>...

> From our limited information it appears that everywhere in the world, after some vehicle broke the speed barrier of 15 mph, time scarcity related to traffic began to grow. After industry had reached this threshold of per capita output, transport made of man a new kind of waif: a being constantly absent from a destination he cannot reach on his own but must attain within the day.

https://www.davidtinapple.com/illich/1973_energy_equity.html