So in order to discourage crossing at non-official crossings, the only answer is passive barriers.

Completely visible barriers would do the trick.

You've somehow, again, managed to miss the point: the purpose was not just deterrence, the purpose was to hide them and cause unexpected harm. I'm not using booby trap to evoke any legality relating to the word; I'm using the word to evoke the horrendously inhumane use of hidden weapons meant to cause harm to those who accidentally stumble upon them.

You're defending a horrific practice in the guise of it being a necessary evil, when in all actuality, it's just one horrific out of many not-horrific implementations of something that you're overtly in favor of.