To look at it another way, it makes the road safer for everyone regardless of the attentiveness and skill of any particular driver. Surely, that's a reasonable trade off in areas with a lot of vulnerable pedestrians.

What do you mean by penalising?

If a road has a 40km/h speed limit, and then road features designed for 40km/h are added, what's the penalty? The point is to require the driver to focus, which a good driver does anyway.

I am not sure how this is a penalty. It's not like it's more physically difficult to drive slower (or faster).

This isn't really true. Road design is more complicated than people think. There's actually a table (I think in the green book) that gives you what are essentially coefficients for drivers "subconscious" speed, given certain design features. Meaning that reducing lane width, shoulder width, or even sign size will cause a driver to "naturally" drive slower. Setting the speed limit outside of this natural range happens all the time, usually in good faith, but results in speed traps.

So maybe not "physically" difficult, but it certainly takes a mental effort.

Don't speed limits on city streets do the same?

A practical answer is “no”. Speed limits were set when cars really couldn’t safely do any more. But, modern cars have better acceleration and braking and suspension. They remain “comfortably” within their handling capabilities even when 30% over the speed limit. (Estimates are my opinion)

One of the article's main points is that when cars drive faster, they are much more likely to kill pedestrians in an accident. So even if modern engineering may allow drivers to stay in control of their cars at higher speeds, cars still become more lethal to non-drivers.

Totally agree - I was more saying that the roads themselves do nothing to control speeds. Or, in a modern car it is easy to speed without realizing it.