You've also got dynamic loading to comsider, eg if a truck full of sand is driving across bridge and slams on the brakes, thats a nuch different impulse than just driving across normally.

Any old schmuck can make a structure with a 10x safety factor. The thing is making the safety factor as low as reasonably possible to minimize costs. If there's a regulation that says 3x minimum, you're probably aiming for 3x. Which is why those regulations are important, I guess.

Source: I write code for a living, don't listen to anything I say

https://safetyculture.com/topics/factor-of-safety/#typical-overall-factors-of-safety-1

Yep, I typically throw more steel at structures than required. If it 'looks' strong enough it is probably twice as strong as it needs to be. There are certain things like walkways that can meet design codes, but it would be bouncy and unnerving to walk on. Beefing up the structure also mitigates the 200lb ape factor i.e. more likely to stay standing if someone drives a forklift into it.